Automate full PR lifecycle (auto-approve and auto-merge)

[MariusStorhaug]

Context

The MVP requires a human reviewer to approve and merge managed-file PRs. For a fully hands-off experience, the process should be automated end-to-end so that repo maintainers receive updates without needing to take any action.

Proposal

Automate the full PR lifecycle so that managed-file PRs are approved and merged without manual intervention. This involves:

1. Marking PRs as ready for review — After the PR is created (and optionally processed by the Copilot agent per Use GitHub Copilot CLI to automate PR title, description, and labels #2), automatically transition the PR from draft to ready for review.
2. Auto-approval — A second GitHub App (e.g., PSModule's Revo) auto-approves the PR once it is ready for review and required checks are green. A separate app is needed because GitHub does not allow the same identity that creates a PR to also approve it.
3. Auto-merge — Enable auto-merge on the PR at creation time so it merges automatically once approved and required checks pass.

Design considerations

• Requires a second GitHub App with pull_requests: write permission for approval.
• The auto-approve mechanism could be implemented as a reusable workflow, an org-level workflow, or a webhook handler.
• Need to decide on the trigger: pull_request: [ready_for_review], check_suite: completed, or a combination.
• Should only trigger for PRs from the managed-files/update branch created by PSModule's Custo.

Dependencies

• MVP distribution mechanism must be in place first (issue [MVP] File distribution service to organization repos #1).
• Optional: Copilot CLI integration (Use GitHub Copilot CLI to automate PR title, description, and labels #2) can enhance the PR metadata before the full automation kicks in.