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/git-flow/skills/commit-conventions.md

@@ -0,0 +1,95 @@
+# Commit Conventions
+
+## Purpose
+
+Defines conventional commit message format for consistent, parseable commit history.
+
+## When to Use
+
+- Generating commit messages in `/commit`
+- Validating user-provided commit messages
+- Explaining commit format to users
+
+## Message Format
+
+```
+<type>(<scope>): <description>
+
+[optional body]
+
+[optional footer]
+```
+
+## Commit Types
+
+| Type | Purpose | Example Scope |
+|------|---------|---------------|
+| `feat` | New feature | `auth`, `api`, `ui` |
+| `fix` | Bug fix | `login`, `validation` |
+| `docs` | Documentation only | `readme`, `api-docs` |
+| `style` | Formatting, whitespace | `lint`, `format` |
+| `refactor` | Code change (no bug fix, no feature) | `auth-module` |
+| `perf` | Performance improvement | `query`, `cache` |
+| `test` | Adding/updating tests | `unit`, `e2e` |
+| `chore` | Maintenance tasks | `deps`, `build` |
+| `build` | Build system or dependencies | `webpack`, `npm` |
+| `ci` | CI configuration | `github-actions` |
+
+## Scope Detection
+
+Derive scope from changed files:
+- `src/auth/*` -> `auth`
+- `src/api/*` -> `api`
+- `tests/*` -> `test`
+- `docs/*` -> `docs`
+- Multiple directories -> most significant or omit
+
+## Examples
+
+**Feature commit:**
+```
+feat(auth): add password reset flow
+
+Implement forgot password with email verification.
+Includes rate limiting (5 attempts/hour) and 24h token expiration.
+
+Closes #123
+```
+
+**Bug fix:**
+```
+fix(ui): resolve button alignment on mobile
+
+The submit button was misaligned on screens < 768px.
+Added responsive flex rules.
+```
+
+**Maintenance:**
+```
+chore(deps): update dependencies
+
+- typescript 5.3 -> 5.4
+- react 18.2 -> 18.3
+- node 18 -> 20 (LTS)
+```
+
+## Footer Conventions
+
+| Footer | Purpose |
+|--------|---------|
+| `Closes #123` | Auto-close issue |
+| `Refs #123` | Reference without closing |
+| `BREAKING CHANGE:` | Breaking change description |
+| `Co-Authored-By:` | Credit co-author |
+
+## Co-Author Footer
+
+When Claude assists with commits:
+```
+Co-Authored-By: Claude <noreply@anthropic.com>
+```
+
+## Related Skills
+
+- skills/branch-naming.md
+- skills/git-safety.md