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/coverage-calculation.md

@@ -0,0 +1,131 @@
+---
+name: coverage-calculation
+description: Documentation coverage metrics, language patterns, and thresholds
+---
+
+# Coverage Calculation
+
+## Purpose
+
+Defines how to calculate documentation coverage and thresholds.
+
+## When to Use
+
+- **doc-coverage**: Full coverage analysis
+- **doc-audit**: Completeness checks
+
+---
+
+## Coverage Formula
+
+```
+Coverage = (Documented Items / Total Items) * 100
+```
+
+---
+
+## Documentable Items by Language
+
+### Python
+- Functions (`def`)
+- Classes
+- Methods
+- Module-level docstrings
+
+### JavaScript/TypeScript
+- Functions (`function`, arrow functions)
+- Classes
+- Methods
+- JSDoc comments (`/** */`)
+
+### Go
+- Functions
+- Types
+- Methods
+- Package comments (`//` above declaration)
+
+### Rust
+- Functions
+- Structs/Enums
+- Impl blocks
+- Doc comments (`///`)
+
+---
+
+## Language Detection
+
+| Extension | Language | Doc Format |
+|-----------|----------|------------|
+| .py | Python | Docstrings (`"""`) |
+| .js, .ts | JavaScript/TypeScript | JSDoc (`/** */`) |
+| .go | Go | `//` comments above |
+| .rs | Rust | `///` doc comments |
+| .rb | Ruby | `#` comments, YARD |
+| .java | Java | Javadoc (`/** */`) |
+
+---
+
+## Coverage Levels
+
+### Basic
+- Item has any docstring/comment
+- Not empty or placeholder
+
+### Standard
+- Docstring describes purpose
+- Non-trivial content (not just `pass` or `TODO`)
+
+### Complete
+- All parameters documented
+- Return type documented
+- Raises/throws documented
+
+---
+
+## Coverage Thresholds
+
+| Level | Coverage | Description |
+|-------|----------|-------------|
+| Minimal | 60% | Basic documentation exists |
+| Good | 80% | Most public APIs documented |
+| Excellent | 95% | Comprehensive documentation |
+
+---
+
+## Output Format
+
+```
+## Documentation Coverage Report
+
+### Summary
+- Total documentable items: 156
+- Documented: 142
+- Coverage: 91.0%
+
+### By Type
+| Type | Total | Documented | Coverage |
+|------|-------|------------|----------|
+| Functions | 89 | 85 | 95.5% |
+| Classes | 23 | 21 | 91.3% |
+| Methods | 44 | 36 | 81.8% |
+
+### By Directory
+| Path | Total | Documented | Coverage |
+|------|-------|------------|----------|
+| src/api/ | 34 | 32 | 94.1% |
+| src/utils/ | 28 | 28 | 100.0% |
+
+### Undocumented Items
+- [ ] src/api/handlers.py:45 `create_order()`
+- [ ] src/models/user.py:23 `UserModel.validate()`
+```
+
+---
+
+## Exclusion Patterns
+
+Default exclusions:
+- `**/test_*` - Test files
+- `**/*_test.*` - Test files
+- Private members (`_prefixed`) unless `--include-private`
+- Generated code (`**/generated/**`)