docs: add worktree rototill design spec (PRI-974)

Design for detect-and-defer worktree support. Superpowers defers to
native harness worktree systems when available, falls back to manual
git worktree creation when not. Covers Phases 0-2: detection, consent,
native tool preference, finishing state detection, and three bug fixes
(#940, #999, #238).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-04-06-worktree-rototill-design.md

@@ -0,0 +1,279 @@
+# Worktree Rototill: Detect-and-Defer
+
+**Date:** 2026-04-06
+**Status:** Draft
+**Ticket:** PRI-974
+**Subsumes:** PRI-823 (Codex App compatibility)
+
+## Problem
+
+Superpowers is opinionated about worktree management — specific paths (`.worktrees/<branch>`), specific commands (`git worktree add`), specific cleanup (`git worktree remove`). Meanwhile, Claude Code, Codex App, Gemini CLI, and Cursor all provide native worktree support with their own paths, lifecycle management, and cleanup.
+
+This creates three failure modes:
+
+1. **Duplication** — on Claude Code, the skill does what `EnterWorktree`/`ExitWorktree` already does
+2. **Conflict** — on Codex App, the skill tries to create worktrees inside an already-managed worktree
+3. **Phantom state** — skill-created worktrees at `.worktrees/` are invisible to the harness; harness-created worktrees at `.claude/worktrees/` are invisible to the skill
+
+For harnesses without native support (Codex CLI, OpenCode, Copilot standalone), superpowers fills a real gap. The skill shouldn't go away — it should get out of the way when native support exists.
+
+## Goals
+
+1. Defer to native harness worktree systems when they exist
+2. Continue providing worktree support for harnesses that lack it
+3. Fix three known bugs in finishing-a-development-branch (#940, #999, #238)
+4. Make worktree creation opt-in rather than mandatory (#991)
+5. Replace hardcoded `CLAUDE.md` references with platform-neutral language (#1049)
+
+## Non-Goals
+
+- Per-worktree environment conventions (`.worktree-env.sh`, port offsetting) — Phase 4
+- PreToolUse hooks for path enforcement — Phase 4
+- Multi-repo worktree documentation — Phase 4
+- Brainstorming checklist changes for worktrees — Phase 4
+- `.superpowers-session.json` metadata tracking (interesting PR #997 idea, not needed for v1)
+- Hooks symlinking into worktrees (PR #965 idea, separate concern)
+
+## Design Principles
+
+### Detect state, not platform
+
+Use `GIT_DIR != GIT_COMMON` to determine "am I already in a worktree?" rather than sniffing environment variables to identify the harness. This is a stable git primitive (since git 2.5, 2015), works universally across all harnesses, and requires zero maintenance as new harnesses appear.
+
+### Declarative intent, prescriptive fallback
+
+The skill describes the goal ("ensure work happens in an isolated workspace") and defers to native tools when available. It prescribes specific git commands only as a fallback for harnesses without native worktree support. Structurally, the native-tool path (Step 1a) comes first and is short; the git fallback (Step 1b) comes second and is long. This prevents agents from anchoring on the detailed fallback and skipping the preferred path.
+
+### Provenance-based ownership
+
+Whoever creates the worktree owns its cleanup. If the harness created it, superpowers doesn't touch it. If superpowers created it (via git fallback), superpowers cleans it up. The heuristic: if the worktree lives under `.worktrees/`, superpowers owns it. Anything else (`.claude/worktrees/`, `~/.codex/worktrees/`, `.gemini/worktrees/`) belongs to the harness.
+
+## Design
+
+### 1. `using-git-worktrees` SKILL.md Rewrite
+
+The skill gains three new steps before creation and simplifies the creation flow.
+
+#### Step 0: Detect Existing Isolation
+
+```bash
+GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+BRANCH=$(git branch --show-current)
+```
+
+Three outcomes:
+
+| Condition | Meaning | Action |
+|-----------|---------|--------|
+| `GIT_DIR == GIT_COMMON` | Normal repo checkout | Proceed to Step 0.5 |
+| `GIT_DIR != GIT_COMMON`, named branch | Already in a linked worktree | Skip to Step 3 (project setup). Report: "Already in isolated workspace at `<path>` on branch `<name>`." |
+| `GIT_DIR != GIT_COMMON`, detached HEAD | Externally managed worktree (e.g., Codex App sandbox) | Skip to Step 3. Report: "Already in isolated workspace at `<path>` (detached HEAD, externally managed)." |
+
+Step 0 does not care who created the worktree or which harness is running. A worktree is a worktree regardless of origin.
+
+#### Step 0.5: Consent
+
+When Step 0 finds no existing isolation (`GIT_DIR == GIT_COMMON`), ask before creating:
+
+> "Would you like me to set up an isolated worktree? This protects your current branch from changes. (y/n)"
+
+If yes, proceed to Step 1. If no, work in place — skip to Step 3 with no worktree.
+
+This step is skipped entirely when Step 0 detects existing isolation (no point asking about what already exists).
+
+#### Step 1a: Native Tools (preferred)
+
+> If your platform provides a worktree or workspace-isolation tool, use it. You know your own toolkit — the skill does not need to name specific tools. Native tools handle directory placement, branch creation, and cleanup automatically.
+
+After using a native tool, skip to Step 3 (project setup).
+
+This section is deliberately short (3 lines). Agents already know their available tools. Listing examples would risk agents attempting tools that don't exist on their platform.
+
+#### Step 1b: Git Worktree Fallback
+
+When no native tool is available, create a worktree manually.
+
+**Directory selection** (priority order):
+1. Check for existing `.worktrees/` or `worktrees/` directory — if found, use it. If both exist, `.worktrees/` wins.
+2. Check the project's agent instruction file (CLAUDE.md, GEMINI.md, AGENTS.md, .cursorrules, or equivalent) for a worktree directory preference.
+3. Default to `.worktrees/`.
+
+No interactive directory selection prompt. No global path option (`~/.config/superpowers/worktrees/` is dropped — no demonstrated user demand, adds cleanup and cross-device complexity).
+
+**Safety verification** (project-local directories only):
+
+```bash
+git check-ignore -q .worktrees 2>/dev/null
+```
+
+If not ignored, add to `.gitignore` and commit before proceeding.
+
+**Create:**
+
+```bash
+git worktree add "$path" -b "$BRANCH_NAME"
+cd "$path"
+```
+
+**Sandbox fallback:** If `git worktree add` fails with a permission error, treat as a restricted environment. Skip creation, work in current directory, proceed to Step 3.
+
+#### Steps 3-4: Project Setup and Baseline Tests (unchanged)
+
+Regardless of which path created the workspace (Step 0 detected existing, Step 1a native tool, Step 1b git fallback, or no worktree at all), execution converges:
+
+- **Step 3:** Auto-detect and run project setup (`npm install`, `cargo build`, `pip install`, `go mod download`, etc.)
+- **Step 4:** Run the test suite. If tests fail, report failures and ask whether to proceed.
+
+### 2. `finishing-a-development-branch` SKILL.md Rewrite
+
+The finishing skill gains environment detection and fixes three bugs.
+
+#### Step 1: Verify Tests (unchanged)
+
+Run the project's test suite. If tests fail, stop. Don't offer completion options.
+
+#### Step 1.5: Detect Environment (new)
+
+Re-run the same detection as Step 0 in creation:
+
+```bash
+GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+```
+
+Three paths:
+
+| State | Menu | Cleanup |
+|-------|------|---------|
+| `GIT_DIR == GIT_COMMON` (normal repo) | Standard 4 options | No worktree to clean up |
+| `GIT_DIR != GIT_COMMON`, named branch | Standard 4 options | Provenance-based (see Step 5) |
+| `GIT_DIR != GIT_COMMON`, detached HEAD | Reduced menu: push as new branch + PR, keep as-is, discard | No merge options (can't merge from detached HEAD) |
+
+#### Step 2: Determine Base Branch (unchanged)
+
+#### Step 3: Present Options
+
+**Normal repo and named-branch worktree:**
+
+1. Merge back to `<base-branch>` locally
+2. Push and create a Pull Request
+3. Keep the branch as-is (I'll handle it later)
+4. Discard this work
+
+**Detached HEAD:**
+
+1. Push as new branch and create a Pull Request
+2. Keep as-is (I'll handle it later)
+3. Discard this work
+
+#### Step 4: Execute Choice
+
+**Option 1 (Merge locally):**
+
+```bash
+# Get main repo root for CWD safety (Bug #238 fix)
+MAIN_ROOT=$(git -C "$(git rev-parse --git-common-dir)/.." rev-parse --show-toplevel)
+
+# Remove worktree BEFORE deleting branch (Bug #999 fix)
+cd "$MAIN_ROOT"
+git worktree remove "$WORKTREE_PATH"  # only if superpowers owns it
+
+# Now safe to work with branches
+git checkout <base-branch>
+git pull
+git merge <feature-branch>
+<run tests>
+git branch -d <feature-branch>
+```
+
+**Option 2 (Create PR):**
+
+Push branch, create PR. Do NOT clean up worktree — user needs it for PR iteration. (Bug #940 fix: remove contradictory "Then: Cleanup worktree" prose.)
+
+**Option 3 (Keep as-is):** No action.
+
+**Option 4 (Discard):** Require typed "discard" confirmation. Then remove worktree (if superpowers owns it), force-delete branch.
+
+#### Step 5: Cleanup (updated)
+
+```
+if GIT_DIR == GIT_COMMON:
+    # Normal repo, no worktree to clean up
+    done
+
+if worktree path is under .worktrees/:
+    # Superpowers created it — we own cleanup
+    cd to main repo root       # Bug #238 fix
+    git worktree remove <path>
+
+else:
+    # Harness created it — hands off
+    # If platform provides a workspace-exit tool, use it
+    # Otherwise, leave the worktree in place
+```
+
+Cleanup only runs for Options 1 and 4. Options 2 and 3 always preserve the worktree. (Bug #940 fix.)
+
+### 3. Integration Updates
+
+#### `subagent-driven-development` and `executing-plans`
+
+Both currently list `using-git-worktrees` as REQUIRED in their integration sections. Change to:
+
+> `using-git-worktrees` — Ensures isolated workspace (creates one or verifies existing)
+
+The skill itself now handles consent (Step 0.5) and detection (Step 0), so calling skills don't need to gate or prompt.
+
+#### `writing-plans`
+
+Remove the stale claim "should be run in a dedicated worktree (created by brainstorming skill)." Brainstorming is a design skill and does not create worktrees. The worktree prompt happens at execution time via `using-git-worktrees`.
+
+### 4. Platform-Neutral Instruction File References
+
+All instances of hardcoded `CLAUDE.md` in worktree-related skills are replaced with:
+
+> "your project's agent instruction file (CLAUDE.md, GEMINI.md, AGENTS.md, .cursorrules, or equivalent)"
+
+This applies to directory preference checks in Step 1b.
+
+## Bug Fixes (bundled)
+
+| Bug | Problem | Fix | Location |
+|-----|---------|-----|----------|
+| #940 | Option 2 prose says "Then: Cleanup worktree (Step 5)" but quick reference says keep it. Step 5 says "For Options 1, 2, 4" but Common Mistakes says "Options 1 and 4 only." | Remove cleanup from Option 2. Step 5 applies to Options 1 and 4 only. | finishing SKILL.md |
+| #999 | Option 1 deletes branch before removing worktree. `git branch -d` can fail because worktree still references the branch. | Reorder: remove worktree first, then delete branch. | finishing SKILL.md |
+| #238 | `git worktree remove` fails silently if CWD is inside the worktree being removed. | Add CWD guard: `cd` to main repo root before `git worktree remove`. | finishing SKILL.md |
+
+## Issues Resolved
+
+| Issue | Resolution |
+|-------|-----------|
+| #940 | Direct fix (Bug #940) |
+| #991 | Opt-in consent in Step 0.5 |
+| #918 | Step 0 detection + Step 1.5 finishing detection |
+| #1009 | Step 0 detects harness-created worktrees and defers |
+| #999 | Direct fix (Bug #999) |
+| #238 | Direct fix (Bug #238) |
+| #1049 | Platform-neutral instruction file references |
+| #279 | Solved by detect-and-defer — native paths respected because we don't override them |
+| #574 | Partially addressed: consent prompt replaces the implicit handoff gap. Full fix (brainstorming checklist step) deferred to Phase 4 |
+
+## Risks
+
+### Step 1a agent anchoring
+
+The highest-risk element. Step 1a is deliberately short (3 lines) to prevent agents from anchoring on it over the detailed Step 1b fallback. However, the inverse risk exists: agents may skip the short Step 1a and go straight to the detailed Step 1b. Mitigation: TDD validation using Claude Code's native worktree support (the richest available), with RED/GREEN tests confirming agents use native tools when available.
+
+### Provenance heuristic
+
+The `.worktrees/ = ours, anything else = hands off` heuristic works for every current harness. If a future harness adopts `.worktrees/` as its convention, we'd have a false positive (superpowers tries to clean up a harness-owned worktree). Low risk — every harness uses branded paths — but worth noting.
+
+### Detached HEAD finishing
+
+The reduced menu for detached HEAD worktrees (no merge option) is correct for Codex App's sandbox model. If a user is in detached HEAD for another reason, the reduced menu still makes sense — you genuinely can't merge from detached HEAD without creating a branch first.
+
+## Future Work (not in this spec)
+
+- **Phase 3 remainder:** `$TMPDIR` directory option (#666), setup docs for caching and env inheritance (#299)
+- **Phase 4:** PreToolUse hooks for path enforcement (#1040), per-worktree env conventions (#597), brainstorming checklist worktree step (#574), multi-repo documentation (#710)