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/code-sentinel/skills/dry-run-workflow.md

@@ -0,0 +1,99 @@
+---
+description: Workflow for previewing changes safely before applying them
+---
+
+# Dry Run Workflow Skill
+
+## Overview
+
+Dry run mode analyzes code and shows proposed changes without modifying files. Essential for reviewing impact before committing to changes.
+
+## Opportunity Scoring
+
+Rate each refactoring opportunity on three dimensions:
+
+### Impact Score (1-5)
+| Score | Meaning | Example |
+|-------|---------|---------|
+| 5 | Major improvement | Cyclomatic complexity 15 -> 3 |
+| 4 | Significant improvement | Function 50 lines -> 15 lines |
+| 3 | Moderate improvement | Better naming, clearer structure |
+| 2 | Minor improvement | Code style modernization |
+| 1 | Cosmetic only | Formatting changes |
+
+### Risk Score (1-5)
+| Score | Meaning | Example |
+|-------|---------|---------|
+| 5 | Very high risk | Changes to core business logic |
+| 4 | High risk | Modifies shared utilities |
+| 3 | Moderate risk | Changes function signatures |
+| 2 | Low risk | Internal implementation only |
+| 1 | Minimal risk | Pure functions, no side effects |
+
+### Effort Score (1-5)
+| Score | Meaning | Example |
+|-------|---------|---------|
+| 5 | Major effort | Requires architecture changes |
+| 4 | Significant effort | Many files affected |
+| 3 | Moderate effort | Multiple related changes |
+| 2 | Low effort | Single file, clear scope |
+| 1 | Trivial | Automated transformation |
+
+## Priority Calculation
+
+```
+Priority = (Impact * 2) - Risk - (Effort * 0.5)
+```
+
+| Priority Range | Recommendation |
+|---------------|----------------|
+| > 5 | Recommended - do it |
+| 3-5 | Optional - consider it |
+| < 3 | Skip - not worth it |
+
+## Output Format
+
+### Recommended Section
+High impact, low risk opportunities:
+```
+1. **pattern-name** at file:lines
+   - Description of the change
+   - Impact: High/Medium/Low (specific metric improvement)
+   - Risk: Low/Medium/High (why)
+   - Run: `/refactor <target> --pattern=<pattern>`
+```
+
+### Optional Section
+Lower priority opportunities grouped by type.
+
+### Summary
+- Count of recommended vs optional
+- Estimated overall improvement percentage
+- Any blockers or dependencies
+
+## Dependency Detection
+
+Before recommending changes, check for:
+
+1. **Test Coverage** - Does this code have tests?
+2. **Usage Scope** - Is it used elsewhere?
+3. **Side Effects** - Does it modify external state?
+4. **Breaking Changes** - Will it change public API?
+
+Flag dependencies in output:
+```
+Note: This refactoring requires updating 3 callers:
+  - src/api/handlers.py:45
+  - src/cli/commands.py:78
+  - tests/test_handlers.py:23
+```
+
+## Safety Checklist
+
+Before recommending any change:
+
+- [ ] All affected code locations identified
+- [ ] No breaking API changes without flag
+- [ ] Test coverage assessed
+- [ ] Side effects documented
+- [ ] Rollback path clear (git)