bug: Preview branches not automatically archived when PRs are merged/closed

[isshaddad]

Provide environment information

n/a

Describe the bug

Preview branches are not automatically archived when pull requests are merged or closed, despite documentation stating this should happen automatically.

Expected: Preview branches should be automatically archived when PRs are merged/closed.

Actual: Preview branches remain active after PRs are merged/closed, causing users to hit branch limits.

Reproduction repo

n/a

To reproduce

1. Connect repository using GitHub App integration
2. Enable preview deployments
3. Open a pull request (preview branch created automatically)
4. Merge or close the pull request
5. Observed: Preview branch remains active
6. Expected: Preview branch should be automatically archived

Additional information

Auto-archive works with GitHub Actions when the deploy command runs after a PR is merged/closed, but does not work with the GitHub App integration (potentially cause: there's no webhook handler for PR close events)