Add signal handling integration for site cleanup

[namespacebrian]

Summary

• Register cleanup handler with Terminus core when site is created
• Fix broken cleanup() method call to use cleanupPantheonSite()
• Add debug logging for cleanup handler registration and execution
• Ensure sites are properly cleaned up when interrupted by SIGINT/SIGTERM

Dependencies

⚠️ This PR depends on pantheon-systems/terminus#2731 being merged first.

The terminus-core PR adds the signal handling infrastructure that this plugin integrates with. Without that PR, the registerCleanupHandler() method and getInstance() static method won't be available.

How it works

1. When a site is successfully created, the plugin registers a cleanup handler with the core Terminus instance
2. If the user presses Ctrl+C (SIGINT) or the process receives SIGTERM, the core signal handler calls all registered cleanup handlers
3. The plugin's cleanup handler calls cleanupPantheonSite() to delete the partially created site and clean up VCS records

Test plan

• Verify the plugin builds and loads without the core PR (should work but cleanup won't register)
• Test with the core PR: run site:create and press Ctrl+C during creation
• Verify the site gets cleaned up properly when interrupted
• Check debug logs show the cleanup handler registration and execution