Create a shared CleanupPreviewDeployment action that unifies PR cleanup logic across all Git providers. Previously, GitHub had comprehensive cleanup (cancels active deployments, kills helper containers, removes all PR containers), while GitLab, Bitbucket, and Gitea only did basic cleanup (delete preview record and remove one container by name).
This fix ensures all providers properly clean up orphaned PR containers when a PR is closed/merged, preventing security issues and resource waste. Also fixes early return bug in GitLab webhook handler.
Fixes#2610🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
