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/merge-workflow.md

@@ -0,0 +1,124 @@
+# Merge Workflow
+
+## Purpose
+
+Defines merge strategies, conflict resolution approaches, and post-merge cleanup procedures.
+
+## When to Use
+
+- Merging feature branches in `/commit-merge`
+- Resolving conflicts during sync operations
+- Cleaning up after successful merges
+
+## Merge Strategies
+
+### 1. Merge Commit (Default)
+
+```bash
+git merge <branch> --no-ff
+```
+
+- **Preserves history** - All commits visible
+- **Creates merge commit** - Clear merge point
+- **Best for:** Feature branches, team workflows
+
+### 2. Squash and Merge
+
+```bash
+git merge <branch> --squash
+git commit -m "feat: complete feature"
+```
+
+- **Single commit** - Clean main branch history
+- **Loses individual commits** - Combined into one
+- **Best for:** PR workflows, many small commits
+
+### 3. Rebase
+
+```bash
+git checkout <branch>
+git rebase <target>
+git checkout <target>
+git merge <branch> --ff-only
+```
+
+- **Linear history** - No merge commits
+- **Rewrites history** - Changes commit hashes
+- **Best for:** Personal branches, clean history
+
+## Standard Merge Procedure
+
+```bash
+# 1. Switch to target branch
+git checkout <target>
+
+# 2. Pull latest changes
+git pull origin <target>
+
+# 3. Merge feature branch
+git merge <feature-branch> [--squash] [--no-ff]
+
+# 4. Push merged result
+git push origin <target>
+```
+
+## Conflict Resolution
+
+When conflicts occur:
+
+```
+Conflicts detected while merging.
+
+Conflicting files:
+- src/auth/login.ts
+- src/auth/types.ts
+
+Options:
+1. Open conflict resolution (guided)
+2. Abort merge (keep local state)
+3. Accept all theirs (loses your changes in conflicts)
+4. Accept all ours (ignores upstream in conflicts)
+```
+
+### Manual Resolution Steps
+
+1. Open conflicting file
+2. Find conflict markers: `<<<<<<<`, `=======`, `>>>>>>>`
+3. Edit to desired state
+4. Remove markers
+5. Stage resolved file: `git add <file>`
+6. Continue: `git merge --continue` or `git rebase --continue`
+
+## Post-Merge Cleanup
+
+After successful merge:
+
+```
+Merge complete. Delete the feature branch?
+1. Yes, delete local and remote (Recommended)
+2. Delete local only
+3. Keep the branch
+```
+
+### Cleanup Commands
+
+```bash
+# Delete local branch
+git branch -d <branch>
+
+# Delete remote branch
+git push origin --delete <branch>
+```
+
+## Pre-Merge Checks
+
+Before merging:
+1. **Verify target exists** - `git branch -a | grep <target>`
+2. **Check for uncommitted changes** - `git status`
+3. **Preview conflicts** - `git merge --no-commit --no-ff <branch>`
+4. **Abort preview** - `git merge --abort`
+
+## Related Skills
+
+- skills/git-safety.md
+- skills/sync-workflow.md