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/doc-guardian/skills/doc-patterns.md

@@ -0,0 +1,67 @@
+---
+name: doc-patterns
+description: Common documentation structures and patterns
+---
+
+# Documentation Patterns
+
+## Purpose
+
+Defines common documentation file structures and their contents.
+
+## When to Use
+
+- **doc-audit**: Understanding what to check in each doc type
+- **doc-coverage**: Identifying documentation locations
+
+---
+
+## README.md Patterns
+
+Typical sections:
+- **Installation**: Version requirements, dependencies
+- **Usage**: Function calls, CLI commands
+- **Configuration**: Environment vars, config files
+- **API**: Endpoint references
+
+---
+
+## CLAUDE.md Patterns
+
+Typical sections:
+- **Project Context**: Tech stack versions
+- **File Structure**: Directory layout
+- **Commands**: Available operations
+- **Workflows**: Process descriptions
+
+---
+
+## Code Documentation
+
+### Docstrings
+- Function signatures
+- Parameters and types
+- Return values
+- Raised exceptions
+
+### Type Hints
+- Should match docstring types
+- Verify consistency
+
+### Inline Comments
+- References to other code
+- TODO markers
+- Warning notes
+
+---
+
+## File Inventory
+
+Standard documentation files to check:
+- `README.md` (root and subdirectories)
+- `CLAUDE.md`
+- `CONTRIBUTING.md`
+- `CHANGELOG.md`
+- `docs/**/*.md`
+- API documentation
+- Configuration references