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

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+---
+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)

plugins/code-sentinel/skills/refactoring-patterns.md

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+---
+description: Code refactoring patterns and techniques for improving structure and maintainability
+---
+
+# Refactoring Patterns Skill
+
+## Structure Patterns
+
+| Pattern | Description | When to Use |
+|---------|-------------|-------------|
+| `extract-method` | Extract code block into named function | Long functions, repeated logic |
+| `extract-class` | Move related methods to new class | Class doing too much |
+| `inline` | Inline trivial function/variable | Over-abstracted code |
+| `rename` | Rename with all references updated | Unclear naming |
+| `move` | Move function/class to different module | Misplaced code |
+
+### extract-method Example
+```python
+# BEFORE
+def process_order(order):
+    # Validate (extract this)
+    if not order.items:
+        raise ValueError("Empty order")
+    if order.total < 0:
+        raise ValueError("Invalid total")
+    # Process
+    for item in order.items:
+        inventory.reserve(item)
+    return create_invoice(order)
+
+# AFTER
+def validate_order(order):
+    if not order.items:
+        raise ValueError("Empty order")
+    if order.total < 0:
+        raise ValueError("Invalid total")
+
+def process_order(order):
+    validate_order(order)
+    for item in order.items:
+        inventory.reserve(item)
+    return create_invoice(order)
+```
+
+## Simplification Patterns
+
+| Pattern | Description | When to Use |
+|---------|-------------|-------------|
+| `simplify-conditional` | Flatten nested if/else | Deep nesting (>3 levels) |
+| `remove-dead-code` | Delete unreachable code | After refactoring |
+| `consolidate-duplicate` | Merge duplicate code blocks | DRY violations |
+| `decompose-conditional` | Break complex conditions into named parts | Long boolean expressions |
+
+### simplify-conditional Example
+```python
+# BEFORE
+if user:
+    if user.active:
+        if user.has_permission:
+            do_action()
+
+# AFTER (guard clauses)
+if not user:
+    return
+if not user.active:
+    return
+if not user.has_permission:
+    return
+do_action()
+```
+
+## Modernization Patterns
+
+| Pattern | Description | When to Use |
+|---------|-------------|-------------|
+| `use-comprehension` | Convert loops to list/dict comprehensions | Simple transformations |
+| `use-pathlib` | Replace os.path with pathlib | File path operations |
+| `use-fstring` | Convert .format() to f-strings | String formatting |
+| `use-typing` | Add type hints | Public APIs, complex functions |
+| `use-dataclass` | Convert class to dataclass | Data-holding classes |
+
+### use-dataclass Example
+```python
+# BEFORE
+class User:
+    def __init__(self, name, email, age):
+        self.name = name
+        self.email = email
+        self.age = age
+
+# AFTER
+@dataclass
+class User:
+    name: str
+    email: str
+    age: int
+```
+
+## Code Smell Detection
+
+| Smell | Indicators | Suggested Pattern |
+|-------|------------|-------------------|
+| Long Method | >20 lines, multiple responsibilities | extract-method |
+| Large Class | >10 methods, low cohesion | extract-class |
+| Primitive Obsession | Many related primitives | use-dataclass |
+| Nested Conditionals | >3 nesting levels | simplify-conditional |
+| Duplicate Code | Copy-pasted blocks | consolidate-duplicate |
+| Dead Code | Unreachable branches | remove-dead-code |
+
+## Metrics
+
+After refactoring, measure improvement:
+
+| Metric | Good Target | Tool |
+|--------|-------------|------|
+| Cyclomatic Complexity | <10 per function | radon, lizard |
+| Function Length | <25 lines | manual count |
+| Class Cohesion | LCOM <0.5 | pylint |
+| Duplication | <3% | jscpd, radon |