Refine review workflow stage gating, notifications, and edge cases

- Suppress backcheck pings to reviewers who haven't reviewed yet
- Handle changes_requested and dismissed review events
- Flip merge gate to pending on post-approval pushes (admin:approve-merge-after-push to re-enable)
- Add assign:<stage> labels to pre-pin reviewer assignments to later stages
- Post one-time guidance on team review requests
- Soften lane-detection failure tone
- Trim verbose transition comments; reference Documentation Guide instead

Author: AdamGohs