Reviews feature enhancements: suppress stale review delivery for older PR heads

[neversettle17-101]

Summary

Track follow-up reviews feature enhancements around stale review delivery and batch hardening.

The new multi-PR batch delivery path should suppress changes_requested results for older PR heads by checking (pr_url, target_sha) against the current PR head before notifying the worker. This issue tracks keeping that invariant explicit across all review delivery paths and future review enhancements.

Problem

Example flow:

1. Trigger review creates a review run for PR chore: scaffold backend/ and frontend/ skeletons for rewrite #1 at sha-old.
2. Reviewer is still working.
3. Worker pushes sha-new.
4. Trigger review creates a newer run/batch for PR chore: scaffold backend/ and frontend/ skeletons for rewrite #1 at sha-new.
5. The old reviewer later submits changes_requested for sha-old.

AO should not notify the worker to address stale feedback for the old SHA.

Desired Behavior

Before sending any worker notification for review feedback:

• Load the current PR state for each candidate changes_requested run.
• Deliver only runs whose (pr_url, target_sha) still match the current PR head.
• Suppress stale runs from worker notification.
• Keep stale runs in history, but do not send them as actionable feedback.

This should remain true for batch delivery and any legacy/single-run delivery path that can still handle empty batch_id rows.

Acceptance Criteria

• A changes_requested review for the current PR head sends a worker notification.
• A changes_requested review for an older SHA does not send a worker notification.
• Mixed batches deliver only current-head changes_requested runs.
• Stale suppressed runs do not block the batch forever.
• Tests cover old-SHA submit after a newer trigger/run exists.

Related Context

This came out of the multi-PR review batching design for issue #395. It is one item in a broader reviews feature enhancements bucket.