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/branch-naming.md

@@ -0,0 +1,97 @@
+# Branch Naming
+
+## Purpose
+
+Defines branch naming conventions and validation rules for consistent repository organization.
+
+## When to Use
+
+- Creating new branches with `/branch-start`
+- Validating branch names
+- Converting descriptions to branch names
+
+## Branch Name Format
+
+```
+<type>/<description>
+```
+
+## Branch Types
+
+| Type | Purpose | Example |
+|------|---------|---------|
+| `feat` | New feature | `feat/user-authentication` |
+| `fix` | Bug fix | `fix/login-timeout` |
+| `chore` | Maintenance | `chore/update-deps` |
+| `docs` | Documentation | `docs/api-reference` |
+| `refactor` | Code restructure | `refactor/auth-module` |
+| `test` | Test additions | `test/auth-coverage` |
+| `perf` | Performance | `perf/query-optimization` |
+| `debug` | Debugging work | `debug/memory-leak` |
+
+## Naming Rules
+
+1. **Lowercase only** - Never use uppercase
+2. **Hyphens for spaces** - Use `-` not `_` or ` `
+3. **No special characters** - Alphanumeric and hyphens only
+4. **Descriptive** - 2-4 words recommended
+5. **Max 50 characters** - Keep concise
+
+## Conversion Algorithm
+
+```
+Input:  "Add User Authentication"
+Output: "feat/add-user-authentication"
+
+Steps:
+1. Lowercase: "add user authentication"
+2. Replace spaces: "add-user-authentication"
+3. Remove special chars: (none to remove)
+4. Add prefix: "feat/add-user-authentication"
+5. Truncate if > 50: (not needed)
+```
+
+## Validation Checks
+
+```
+Branch name validation:
+[x] Lowercase
+[x] Valid prefix (feat/)
+[x] Descriptive (3+ words recommended)
+[ ] Too long (52 chars, max 50)
+
+Suggested: feat/add-user-auth
+```
+
+## Examples
+
+**Valid:**
+```
+feat/add-password-reset
+fix/null-pointer-login
+chore/upgrade-typescript-5
+docs/update-readme
+refactor/simplify-auth
+```
+
+**Invalid:**
+```
+Feature/Add_Password_Reset  (wrong case, underscores)
+fix-bug                      (too vague, no prefix)
+my-branch                    (no type prefix)
+feat/add-new-super-amazing-feature-for-users  (too long)
+```
+
+## Issue-Linked Branches
+
+When working on issues, include issue number:
+```
+feat/123-add-password-reset
+fix/456-login-timeout
+```
+
+## Related Skills
+
+- skills/commit-conventions.md
+- skills/git-safety.md
+- skills/workflow-patterns/branching-strategies.md