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/sync-workflow.md

@@ -0,0 +1,146 @@
+# Sync Workflow
+
+## Purpose
+
+Defines push/pull patterns, rebase strategies, upstream tracking, and stale branch detection.
+
+## When to Use
+
+- Pushing commits in `/commit-push`
+- Full sync operations in `/commit-sync`
+- Detecting and reporting stale branches
+
+## Push Workflow
+
+### First Push (No Upstream)
+
+```bash
+git push -u origin <branch>
+```
+
+Sets upstream tracking for future pushes.
+
+### Subsequent Pushes
+
+```bash
+git push
+```
+
+Pushes to tracked upstream.
+
+### Push After Rebase
+
+```bash
+git push --force-with-lease
+```
+
+Safe force push - fails if remote has new commits.
+
+## Sync with Base Branch
+
+```bash
+# 1. Fetch all with prune
+git fetch --all --prune
+
+# 2. Rebase on base branch
+git rebase origin/<base-branch>
+
+# 3. Push (force if rebased)
+git push --force-with-lease
+```
+
+## Push Conflict Handling
+
+When push fails due to diverged history:
+
+```
+Remote has changes not in your local branch.
+
+Options:
+1. Pull and rebase, then push (Recommended)
+2. Pull and merge, then push
+3. Force push (destructive - requires confirmation)
+4. Cancel and review manually
+```
+
+### Rebase Resolution
+
+```bash
+git pull --rebase origin <branch>
+git push
+```
+
+### Merge Resolution
+
+```bash
+git pull origin <branch>
+git push
+```
+
+## Stale Branch Detection
+
+Find local branches tracking deleted remotes:
+
+```bash
+git branch -vv | grep ': gone]'
+```
+
+### Report Format
+
+```
+Stale local branches (remote deleted):
+  - feat/old-feature (was tracking origin/feat/old-feature)
+  - fix/merged-bugfix (was tracking origin/fix/merged-bugfix)
+
+Run /branch-cleanup to remove these branches.
+```
+
+## Remote Pruning
+
+Remove stale remote-tracking references:
+
+```bash
+git fetch --prune
+```
+
+Or fetch all remotes:
+
+```bash
+git fetch --all --prune
+```
+
+## Sync Status Report
+
+```
+Sync complete:
+
+Local:  feat/password-reset @ abc1234
+Remote: origin/feat/password-reset @ abc1234
+Base:   development @ xyz7890 (synced)
+
+Your branch is up-to-date with development.
+No conflicts detected.
+
+Cleanup:
+  Remote refs pruned: 2
+  Stale local branches: 2 (run /branch-cleanup to remove)
+```
+
+## Tracking Setup
+
+Check tracking status:
+
+```bash
+git branch -vv
+```
+
+Set upstream:
+
+```bash
+git branch --set-upstream-to=origin/<branch> <branch>
+```
+
+## Related Skills
+
+- skills/git-safety.md
+- skills/merge-workflow.md