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/claude-config-maintainer/skills/pre-change-protocol.md

@@ -0,0 +1,83 @@
+# Pre-Change Protocol
+
+This skill defines the mandatory Pre-Change Protocol section that MUST be included in every CLAUDE.md file.
+
+## Why This Is Mandatory
+
+The Pre-Change Protocol prevents the #1 cause of bugs from AI-assisted coding: **incomplete changes where Claude modifies some files but misses others that reference the same code**.
+
+Without this protocol:
+- Claude may rename a function but miss callers
+- Claude may modify a config but miss documentation
+- Claude may update a schema but miss dependent code
+
+## Detection
+
+Search CLAUDE.md for these indicators:
+- Header containing "Pre-Change" or "Before Any Code Change"
+- References to `grep -rn` or impact search
+- Checklist with "Files That Will Be Affected"
+- User verification checkpoint
+
+## Required Section Content
+
+```markdown
+## MANDATORY: Before Any Code Change
+
+**Claude MUST show this checklist BEFORE editing any file:**
+
+### 1. Impact Search Results
+Run and show output of:
+```bash
+grep -rn "PATTERN" --include="*.sh" --include="*.md" --include="*.json" --include="*.py" | grep -v ".git"
+```
+
+### 2. Files That Will Be Affected
+Numbered list of every file to be modified, with the specific change for each.
+
+### 3. Files Searched But Not Changed (and why)
+Proof that related files were checked and determined unchanged.
+
+### 4. Documentation That References This
+List of docs that mention this feature/script/function.
+
+**User verifies this list before Claude proceeds. If Claude skips this, stop immediately.**
+
+### After Changes
+Run the same grep and show results proving no references remain unaddressed.
+```
+
+## Placement
+
+Insert Pre-Change Protocol section:
+- **After:** Critical Rules section
+- **Before:** Common Operations section
+
+## If Missing During Analysis
+
+Flag as **HIGH PRIORITY** issue:
+
+```
+1. [HIGH] Missing Pre-Change Protocol section
+   CLAUDE.md lacks mandatory dependency-check protocol.
+   Impact: Claude may miss file references when making changes,
+   leading to broken dependencies and incomplete updates.
+
+   Recommendation: Add Pre-Change Protocol section immediately.
+   This is the #1 cause of cascading bugs from incomplete changes.
+```
+
+## If Missing During Optimization
+
+**Automatically add the section** at the correct position. This is the highest priority enhancement.
+
+## Variations
+
+The exact wording can vary, but these elements are required:
+
+1. **Search requirement** - Must run grep/search before changes
+2. **Affected files list** - Must enumerate all files to modify
+3. **Non-affected files proof** - Must show what was checked but unchanged
+4. **Documentation check** - Must list referencing docs
+5. **User checkpoint** - Must pause for user verification
+6. **Post-change verification** - Must verify after changes