feat(skills): add rules-distill skill (rebased #561) (#678)

* feat(skills): add rules-distill — extract cross-cutting principles from skills into rules

Applies the skill-stocktake pattern to rules maintenance:
scan skills → extract shared principles → propose rule changes.

Key design decisions:
- Deterministic collection (scan scripts) + LLM judgment (cross-read & verdict)
- 6 verdict types: Append, Revise, New Section, New File, Already Covered, Too Specific
- Anti-abstraction safeguard: 2+ skills evidence, actionable behavior test, violation risk
- Rules full text passed to LLM (no grep pre-filter) for accurate matching
- Never modifies rules automatically — always requires user approval

* fix(skills): address review feedback for rules-distill

Fixes raised by CodeRabbit, Greptile, and cubic:

- Add Prerequisites section documenting skill-stocktake dependency
- Add fallback command when skill-stocktake is not installed
- Fix shell quoting: add IFS= and -r to while-read loops
- Replace hardcoded paths with env var placeholders ($CLAUDE_RULES_DIR, $SKILL_STOCKTAKE_DIR)
- Add json language identifier to code blocks
- Add "How It Works" parent heading for Phase 1/2/3
- Add "Example" section with end-to-end run output
- Add revision.reason/before/after fields to output schema for Revise verdict
- Document timestamp format (date -u +%Y-%m-%dT%H:%M:%SZ)
- Document candidate-id format (kebab-case from principle)
- Use concrete examples in results.json schema

* fix(skills): remove skill-stocktake dependency, add self-contained scripts

Address P1 review feedback:
- Add scan-skills.sh and scan-rules.sh directly in rules-distill/scripts/
  (no external dependency on skill-stocktake)
- Remove Prerequisites section (no longer needed)
- Add cross-batch merge step to prevent 2+ skills requirement
  from being silently broken across batch boundaries
- Fix nested triple-backtick fences (use quadruple backticks)
- Remove head -100 cap (silent truncation)
- Rename "When to Activate" → "When to Use" (ECC standard)
- Remove unnecessary env var placeholders (SKILL.md is a prompt, not a script)

* fix: update skill/command counts in README.md and AGENTS.md

rules-distill added 1 skill + 1 command:
- skills: 108 → 109
- commands: 57 → 58

Updates all count references to pass CI catalog validation.

* fix(skills): address Servitor review feedback for rules-distill

1. Rename SKILL_STOCKTAKE_* env vars to RULES_DISTILL_* for consistency
2. Remove unnecessary observation counting (use_7d/use_30d) from scan-skills.sh
3. Fix header comment: scan.sh → scan-skills.sh
4. Use jq for JSON construction in scan-rules.sh to properly escape
   headings containing special characters (", \)

* fix(skills): address CodeRabbit review — portability and scan scope

1. scan-rules.sh: use jq for error JSON output (proper escaping)
2. scan-rules.sh: replace GNU-only sort -z with portable sort (BSD compat)
3. scan-rules.sh: fix pipefail crash on files without H2 headings
4. scan-skills.sh: scan only SKILL.md files (skip learned/*.md and
   auxiliary docs that lack frontmatter)
5. scan-skills.sh: add portable get_mtime helper (GNU stat/date
   fallback to BSD stat/date)

* fix: sync catalog counts with filesystem (27 agents, 114 skills, 59 commands)

---------

Co-authored-by: Tatsuya Shimomoto <shimo4228@gmail.com>

commands/rules-distill.md

@@ -0,0 +1,11 @@
+---
+description: "Scan skills to extract cross-cutting principles and distill them into rules"
+---
+
+# /rules-distill — Distill Principles from Skills into Rules
+
+Scan installed skills, extract cross-cutting principles, and distill them into rules.
+
+## Process
+
+Follow the full workflow defined in the `rules-distill` skill.