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/skills/security-patterns/SKILL.md

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+---
+description: Security vulnerability patterns and detection rules
+---
+
+# Security Patterns Skill
+
+## Critical Patterns (Always Block)
+
+### SQL Injection
+```python
+# VULNERABLE
+query = f"SELECT * FROM users WHERE id = {user_id}"
+query = "SELECT * FROM users WHERE id = " + user_id
+
+# SAFE
+cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))
+User.objects.filter(id=user_id)
+```
+
+### Command Injection
+```python
+# VULNERABLE
+os.system(f"convert {filename} output.png")
+subprocess.run(cmd, shell=True)
+
+# SAFE
+subprocess.run(["convert", filename, "output.png"], shell=False)
+shlex.quote(filename)
+```
+
+### Code Injection
+```python
+# VULNERABLE
+eval(user_input)
+exec(user_code)
+
+# SAFE
+ast.literal_eval(user_input)  # Only for literals
+# Use sandboxed execution environment
+```
+
+### XSS
+```javascript
+// VULNERABLE
+element.innerHTML = userContent;
+dangerouslySetInnerHTML={{__html: userData}}
+
+// SAFE
+element.textContent = userContent;
+DOMPurify.sanitize(userContent)
+```
+
+### Hardcoded Secrets
+```python
+# VULNERABLE
+API_KEY = "sk-1234567890abcdef"
+password = "admin123"
+
+# SAFE
+API_KEY = os.environ.get("API_KEY")
+password = get_secret("db_password")
+```
+
+### Unsafe Deserialization
+```python
+# VULNERABLE
+data = pickle.loads(user_data)
+config = yaml.load(file)  # yaml.load without Loader
+
+# SAFE
+data = json.loads(user_data)
+config = yaml.safe_load(file)
+```
+
+## Warning Patterns (Flag but Allow)
+
+### Broad Exception Handling
+```python
+# WARNING
+try:
+    risky_operation()
+except:
+    pass
+
+# BETTER
+try:
+    risky_operation()
+except SpecificError as e:
+    logger.error(f"Operation failed: {e}")
+    raise
+```
+
+### Missing Timeout
+```python
+# WARNING
+response = requests.get(url)
+
+# BETTER
+response = requests.get(url, timeout=30)
+```
+
+### Path Traversal Risk
+```python
+# WARNING
+file_path = os.path.join(base_dir, user_filename)
+
+# BETTER
+file_path = os.path.join(base_dir, os.path.basename(user_filename))
+if not file_path.startswith(os.path.abspath(base_dir)):
+    raise ValueError("Invalid path")
+```