fix: update worktree integration references across skills (PRI-974)

Remove REQUIRED language from executing-plans and subagent-driven-development.
Consent and detection now live inside using-git-worktrees itself.
Fix stale 'created by brainstorming' claim in writing-plans.

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

skills/executing-plans/SKILL.md

@@ -65,6 +65,6 @@ After all tasks complete and verified:
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - Ensures isolated workspace (creates one or verifies existing)
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks

skills/subagent-driven-development/SKILL.md

@@ -265,7 +265,7 @@ Done!
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - Ensures isolated workspace (creates one or verifies existing)
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:requesting-code-review** - Code review template for reviewer subagents
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks

skills/writing-plans/SKILL.md

@@ -13,7 +13,7 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 
 **Announce at start:** "I'm using the writing-plans skill to create the implementation plan."
 
-**Context:** This should be run in a dedicated worktree (created by brainstorming skill).
+**Context:** If working in an isolated worktree, it should have been created via the using-git-worktrees skill at execution time.
 
 **Save plans to:** `docs/superpowers/plans/YYYY-MM-DD-<feature-name>.md`
 - (User preferences for plan location override this default)