refactor: extract skills from commands across 8 plugins

Refactored commands to extract reusable skills following the
Commands → Skills separation pattern. Each command is now <50 lines
and references skill files for detailed knowledge.

Plugins refactored:
- claude-config-maintainer: 5 commands → 7 skills
- code-sentinel: 3 commands → 2 skills
- contract-validator: 5 commands → 6 skills
- data-platform: 10 commands → 6 skills
- doc-guardian: 5 commands → 6 skills (replaced nested dir)
- git-flow: 8 commands → 7 skills

Skills contain: workflows, validation rules, conventions,
reference data, tool documentation

Commands now contain: YAML frontmatter, agent assignment,
skills list, brief workflow steps, parameters

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/doc-guardian/skills/staleness-metrics.md

@@ -0,0 +1,127 @@
+---
+name: staleness-metrics
+description: Documentation staleness levels, thresholds, and relationship detection
+---
+
+# Staleness Metrics
+
+## Purpose
+
+Defines how to measure documentation staleness relative to code changes.
+
+## When to Use
+
+- **stale-docs**: Commit-based staleness detection
+- **doc-audit**: Age-based analysis
+
+---
+
+## Staleness Calculation
+
+```
+Commits Behind = Code Commits Since Doc Update
+Days Behind = Days Since Doc Update - Days Since Code Update
+```
+
+---
+
+## Staleness Levels
+
+| Commits Behind | Level | Action |
+|----------------|-------|--------|
+| 0-5 | Fresh | No action needed |
+| 6-10 | Aging | Review recommended |
+| 11-20 | Stale | Update needed |
+| 20+ | Critical | Immediate attention |
+
+---
+
+## Relationship Detection
+
+### 1. Same Directory
+`src/api/README.md` relates to `src/api/**/*`
+
+### 2. Name Matching
+`docs/auth.md` relates to `**/auth.*`, `**/auth/**`
+
+### 3. Explicit Links
+Parse `[link](path)` in docs to find related files
+
+### 4. Import Analysis
+Track which modules are referenced in code examples
+
+---
+
+## Git Commands
+
+```bash
+# Last doc commit
+git log -1 --format="%H %ai" -- docs/api.md
+
+# Last code commit for related files
+git log -1 --format="%H %ai" -- src/api/
+
+# Commits to code since doc update
+git rev-list <doc-commit>..HEAD -- src/api/ | wc -l
+```
+
+---
+
+## Configuration
+
+`.doc-guardian.yml`:
+```yaml
+stale-docs:
+  threshold: 10
+  mappings:
+    - doc: docs/deployment.md
+      code:
+        - Dockerfile
+        - docker-compose.yml
+        - .github/workflows/deploy.yml
+    - doc: ARCHITECTURE.md
+      code:
+        - src/**/*
+  ignore:
+    - CHANGELOG.md
+    - LICENSE
+    - vendor/**
+```
+
+---
+
+## Output Format
+
+```
+## Stale Documentation Report
+
+### Critical (20+ commits behind)
+| File | Last Updated | Commits Behind | Related Code |
+|------|--------------|----------------|--------------|
+| docs/api.md | 2024-01-15 | 34 | src/api/**/* |
+
+### Stale (11-20 commits behind)
+| File | Last Updated | Commits Behind | Related Code |
+|------|--------------|----------------|--------------|
+| README.md | 2024-02-20 | 15 | package.json, src/index.ts |
+
+### Aging (6-10 commits behind)
+| File | Last Updated | Commits Behind | Related Code |
+|------|--------------|----------------|--------------|
+| CONTRIBUTING.md | 2024-03-01 | 8 | .github/*, scripts/* |
+
+### Summary
+- Critical: 1 file
+- Stale: 1 file
+- Aging: 1 file
+- Fresh: 12 files
+- Total documentation files: 15
+```
+
+---
+
+## Exit Codes
+
+- 0: No critical or stale documentation
+- 1: Stale documentation found (for CI)
+- 2: Critical documentation found