feat: add code-sentinel plugin for security scanning and refactoring

Adds security scanning via PreToolUse hooks + refactoring commands:
- PreToolUse hook catches security issues before code is written
- /security-scan command for comprehensive security audit
- /refactor command to apply refactoring patterns
- /refactor-dry command to preview refactoring opportunities
- security-reviewer agent for vulnerability analysis
- refactor-advisor agent for code structure improvements
- security-patterns skill for vulnerability detection rules

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/code-sentinel/commands/refactor-dry.md

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+---
+description: Preview refactoring changes without applying them
+---
+
+# Refactor Dry Run
+
+Analyze and preview refactoring opportunities without making changes.
+
+## Usage
+```
+/refactor-dry <target> [--all]
+```
+
+**Target:** File path, function name, or "." for current file
+**--all:** Show all opportunities, not just recommended
+
+## Process
+
+1. **Scan Target**
+   Analyze code for refactoring opportunities.
+
+2. **Score Opportunities**
+   Each opportunity rated by:
+   - Impact (how much it improves code)
+   - Risk (likelihood of breaking something)
+   - Effort (complexity of the refactoring)
+
+3. **Output**
+```
+## Refactoring Opportunities: src/handlers.py
+
+### Recommended (High Impact, Low Risk)
+
+1. **extract-method** at lines 45-67
+   - Extract order validation logic
+   - Impact: High (reduces complexity from 12 to 4)
+   - Risk: Low (pure function, no side effects)
+   - Run: `/refactor src/handlers.py:45 --pattern=extract-method`
+
+2. **use-dataclass** for OrderInput class
+   - Convert to dataclass with validation
+   - Impact: Medium (reduces boilerplate)
+   - Risk: Low
+   - Run: `/refactor src/models.py:OrderInput --pattern=use-dataclass`
+
+### Optional (Consider Later)
+
+3. **use-fstring** at 12 locations
+   - Modernize string formatting
+   - Impact: Low (readability only)
+   - Risk: None
+
+### Summary
+- 2 recommended refactorings
+- 1 optional improvement
+- Estimated complexity reduction: 35%
+```