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/claude-md-integration.md

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+# Code Sentinel Integration
+
+Add to your project's CLAUDE.md:
+
+## Security & Code Quality
+
+This project uses code-sentinel for security scanning and refactoring.
+
+### Automatic Security Checks
+PreToolUse hooks scan all code changes for:
+- SQL/Command/Code injection
+- XSS vulnerabilities
+- Hardcoded secrets
+- Unsafe deserialization
+
+Critical issues are blocked. Warnings are noted but allowed.
+
+### Commands
+- `/security-scan` - Full project security audit
+- `/refactor <target>` - Apply refactoring pattern
+- `/refactor-dry <target>` - Preview refactoring opportunities
+
+### Severity Levels
+- 🔴 Critical: Must fix immediately
+- 🟠 High: Fix before release
+- 🟡 Medium: Improve when possible