Prevent scheduled review fanout loops

[seonghobae]

Summary

• require workflow_run-triggered merge scheduler jobs to have associated PR metadata before scanning
• prevent central workflow_dispatch review completions from waking a repo-wide open-PR scan
• cancel superseded PR Review Fix Scheduler cron runs so scheduled backlog cannot accumulate

Root cause addressed

Central review workflows dispatched by the scheduler can complete as workflow_dispatch runs without pull_requests metadata. The merge scheduler listened to every completed OpenCode/Strix workflow_run, so those unassociated completions fell back to scanning the whole open PR queue. Combined with cron and stale queued runs, that kept refilling the Actions queue.

Tests

• uv run --python 3.13 pytest -s -q tests/test_required_workflow_queue_contract.py tests/test_pr_review_merge_scheduler.py tests/test_pr_review_fix_scheduler.py
• uv run --python 3.13 python scripts/ci/pr_review_merge_scheduler.py --self-test
• uv run --python 3.13 python scripts/ci/pr_review_fix_scheduler.py --self-test
• workflow YAML parsed with PyYAML
• git diff --check