feat: add security review command for git changes

[novalis78]

Add /security-review command that performs AI-powered security analysis of uncommitted git changes. The command only appears when there are pending changes to review.

Features:

• Automatic detection of git changes (only shows when needed)
• Comprehensive security analysis covering:
  • Security vulnerabilities (SQL injection, XSS, CSRF, etc.)
  • Insecure dependencies or configurations
  • Hardcoded secrets, credentials, or API keys
  • Permission or access control issues
  • Logging of sensitive data
  • Other security concerns
• Context-aware: includes current branch and full diff
• Integrates seamlessly with existing command palette

Implementation:

• Add SecurityReviewMsg type for command handling
• Add hasGitChanges() to detect uncommitted changes
• Add performSecurityReview() to generate security analysis prompt
• Conditionally show command only when git changes exist

🤖 Generated with Claude Code

• [ ] I have read CONTRIBUTING.md.
• [ ] I have created a discussion that was approved by a maintainer (for new features).