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/diff-analysis.md

@@ -0,0 +1,97 @@
+# CLAUDE.md Diff Analysis
+
+This skill defines how to analyze and present CLAUDE.md differences.
+
+## Comparison Modes
+
+| Mode | Command | Description |
+|------|---------|-------------|
+| Working vs HEAD | `/config-diff` | Uncommitted changes |
+| Working vs Commit | `--commit=REF` | Changes since specific point |
+| Commit to Commit | `--from=X --to=Y` | Historical comparison |
+| Branch Comparison | `--branch=NAME` | Cross-branch differences |
+
+## Change Indicators
+
+| Symbol | Meaning |
+|--------|---------|
+| `+` | Line added |
+| `-` | Line removed |
+| `@@` | Location marker (line numbers) |
+| `[MODIFIED]` | Section has changes |
+| `[ADDED]` | New section created |
+| `[REMOVED]` | Section deleted |
+| `[UNCHANGED]` | No changes to section |
+
+## Impact Categories
+
+| Category | Meaning |
+|----------|---------|
+| NEW REQUIREMENT | Claude will need to do something new |
+| REMOVED REQUIREMENT | Claude no longer needs to do something |
+| MODIFIED | Existing behavior changed |
+| NEW RULE | New constraint added |
+| RELAXED RULE | Constraint removed or softened |
+
+## Report Format
+
+```
+CLAUDE.md Diff Report
+=====================
+
+File: /path/to/project/CLAUDE.md
+Comparing: [mode description]
+Commit: [ref] "[message]" (time ago)
+
+Summary:
+- Lines added: N
+- Lines removed: N
+- Net change: +/-N lines
+- Sections affected: N
+
+Section Changes:
+----------------
+
+## Section Name [STATUS]
+  +/- Change description
+
+Detailed Diff:
+--------------
+
+--- CLAUDE.md (before)
++++ CLAUDE.md (after)
+
+@@ -N,M +N,M @@
+ context
+-removed
++added
+ context
+
+Behavioral Impact:
+------------------
+
+These changes will affect Claude's behavior:
+
+N. [CATEGORY] Description of impact
+```
+
+## Section-Focused View
+
+When using `--section=NAME`:
+- Filter diff to only that section
+- Show section-specific statistics
+- Highlight behavioral impact for that area
+
+## Troubleshooting
+
+### No changes detected
+- File matches comparison target
+- Verify comparing correct commits
+
+### File not found in commit
+- CLAUDE.md didn't exist at that point
+- Use `git log -- CLAUDE.md` to find creation
+
+### Not a git repository
+- Command requires git history
+- Initialize git or use file backup comparison