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/refactoring-patterns.md

@@ -0,0 +1,119 @@
+---
+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 |