feat: GStack Learns — per-project self-learning infrastructure (v0.13.4.0) (#622)

* feat: learnings + confidence resolvers — cross-skill memory infrastructure

Three new resolvers for the self-learning system:
- LEARNINGS_SEARCH: tells skills to load prior learnings before analysis
- LEARNINGS_LOG: tells skills to capture discoveries after completing work
- CONFIDENCE_CALIBRATION: adds 1-10 confidence scoring to all review findings

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

* feat: learnings bin scripts — append-only JSONL read/write

gstack-learnings-log: validates JSON, auto-injects timestamp, appends to
~/.gstack/projects/$SLUG/learnings.jsonl. Append-only (no mutation).

gstack-learnings-search: reads/filters/dedupes learnings with confidence
decay (observed/inferred lose 1pt/30d), cross-project discovery, and
"latest winner" resolution per key+type.

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

* feat: learnings count in preamble output

Every skill now prints "LEARNINGS: N entries loaded" during preamble,
making the compounding loop visible to the user.

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

* feat: integrate learnings + confidence into 9 skill templates

Add {{LEARNINGS_SEARCH}}, {{LEARNINGS_LOG}}, and {{CONFIDENCE_CALIBRATION}}
placeholders to review, ship, plan-eng-review, plan-ceo-review, office-hours,
investigate, retro, and cso templates. Regenerated all SKILL.md files.

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

* feat: /learn skill — manage project learnings

New skill for reviewing, searching, pruning, and exporting what gstack
has learned across sessions. Commands: /learn, /learn search, /learn prune,
/learn export, /learn stats, /learn add.

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

* docs: self-learning roadmap — 5-release design doc

Covers: R1 GStack Learns (v0.14), R2 Review Army (v0.15), R3 Smart Ceremony
(v0.16), R4 /autoship (v0.17), R5 Studio (v0.18). Inspired by Compound
Engineering, adapted to GStack's architecture.

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

* test: learnings bin script unit tests — 13 tests, free

Tests gstack-learnings-log (valid/invalid JSON, timestamp injection,
append-only) and gstack-learnings-search (dedup, type/query/limit filters,
confidence decay, user-stated no-decay, malformed JSONL skip).

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

* chore: bump version and changelog (v0.13.4.0)

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

* test: learnings resolver + bin script edge case tests — 21 new tests, free

Adds gen-skill-docs coverage for LEARNINGS_SEARCH, LEARNINGS_LOG, and
CONFIDENCE_CALIBRATION resolvers. Adds bin script edge cases: timestamp
preservation, special characters, files array, sort order, type grouping,
combined filtering, missing fields, confidence floor at 0.

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

* fix: sync package.json version with VERSION file (0.13.4.0)

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

* chore: gitignore .factory/ — generated output, not source

Same pattern as .claude/skills/ and .agents/. These SKILL.md files are
generated from .tmpl templates by gen:skill-docs --host factory.

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

* test: /learn E2E — seed 3 learnings, verify agent surfaces them

Seeds N+1 query pattern, stale cache pitfall, and rubocop preference
into learnings.jsonl, then runs /learn and checks that at least 2/3
appear in the agent's output. Gate tier, ~$0.25/run.

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

---------

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

bin/gstack-learnings-log

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# gstack-learnings-log — append a learning to the project learnings file
+# Usage: gstack-learnings-log '{"skill":"review","type":"pitfall","key":"n-plus-one","insight":"...","confidence":8,"source":"observed"}'
+#
+# Append-only storage. Duplicates (same key+type) are resolved at read time
+# by gstack-learnings-search ("latest winner" per key+type).
+set -euo pipefail
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
+GSTACK_HOME="${GSTACK_HOME:-$HOME/.gstack}"
+mkdir -p "$GSTACK_HOME/projects/$SLUG"
+
+INPUT="$1"
+
+# Validate: input must be parseable JSON
+if ! printf '%s' "$INPUT" | bun -e "JSON.parse(await Bun.stdin.text())" 2>/dev/null; then
+  echo "gstack-learnings-log: invalid JSON, skipping" >&2
+  exit 1
+fi
+
+# Inject timestamp if not present
+if ! printf '%s' "$INPUT" | bun -e "const j=JSON.parse(await Bun.stdin.text()); if(!j.ts) process.exit(1)" 2>/dev/null; then
+  INPUT=$(printf '%s' "$INPUT" | bun -e "
+    const j = JSON.parse(await Bun.stdin.text());
+    j.ts = new Date().toISOString();
+    console.log(JSON.stringify(j));
+  " 2>/dev/null) || true
+fi
+
+echo "$INPUT" >> "$GSTACK_HOME/projects/$SLUG/learnings.jsonl"