Self-driving: dismiss a REQUEST_CHANGES review once the agent has addressed it

Filing via the API, so I can't apply the enhancement label myself — please tag per CONTRIBUTING.

Is your feature request related to a problem?

Please describe.

GitHub does not auto-dismiss a REQUEST_CHANGES review when new commits land — the dismiss_stale_reviews_on_push setting only clears approvals. So when our reviewer requests changes on a self-driving PR and the agent fixes everything, the review still reads as blocking until a human dismisses it by hand.

The PR looks unmergeable when it isn't, which is a bad state for a PR whose whole selling point is that a human only has to click merge.

Describe the solution you'd like

After the agent has addressed every finding of a REQUEST_CHANGES review — and has replied on the threads (PostHog/posthog#79131) — dismiss the review with a message citing the fix commits.

It's one call:

PUT /repos/{owner}/{repo}/pulls/{pull_number}/reviews/{review_id}/dismissals

Describe alternatives you've considered

• Dismissing by hand. Current state. It's a small thing per PR but it's pure overhead, and it's easy to forget — a stale change-request can sit on a PR for days looking like a real blocker.
• Not using REQUEST_CHANGES for bot reviews. Would work but throws away a useful signal for human reviewers.

Additional context

If you'd rather an agent never dismissed a human's review — reasonable — gating this to reviews authored by bots would still cover our case entirely. Ours are all from a bot reviewer.

Depends on PostHog/posthog#79131: dismissing without replying first would be worse than not dismissing.

Debug info

• PostHog Cloud (US)

No file or test is named. Start by tracing the self-driving review workflow and its GitHub API calls, then read PostHog/posthog#79131 before changing the sequence. Done means bot-authored REQUEST_CHANGES reviews are dismissed through the documented dismissals endpoint only after all findings have been addressed and their threads replied to, with fix commits cited.