feat: enhance self-dogfooding setup with security and testing improvements

[AvaPrime]

Summary

This PR enhances the GitGuard self-dogfooding setup with comprehensive security and testing improvements.

Changes

Infrastructure Enhancements

• Pin Docker Images: Updated docker-compose.temporal.yml to use stable versions (Temporal 1.24.4, Postgres 13.15)
• GitHub App Manifest: Updated app.json with proper GitHub App configuration for easy setup

Documentation Improvements

• Enhanced SELF_DOGFOOD.md: Added comprehensive sections covering:
  • GitHub App creation with manifest instructions
  • Webhook safety and security best practices
  • End-to-end smoke testing with three detailed scenarios
  • Verification checklist with 8 key checkpoints
  • Test cleanup procedures

Security & Safety Features

• Webhook Validation: HMAC-SHA256 signature verification
• Tunnel Security: Best practices for ngrok and cloudflared
• Rate Limiting: Monitoring and webhook delivery tracking
• Automated Testing: Built-in smoketest functionality

Testing

• Docker image versions pinned to stable releases
• GitHub App manifest configuration validated
• Documentation includes comprehensive testing scenarios
• PowerShell script already includes smoketest functionality

Impact

This enhancement provides:

• One-command setup for both Linux/macOS and Windows
• Comprehensive testing scenarios to validate policy enforcement
• Security-first approach with proper webhook validation
• Clear next steps for GitHub App creation and configuration