ci(codex-review): add retries, concurrency, fallback model

[alexanderkreidich]

Summary

• Drop synchronize trigger; review only on opened/reopened/ready_for_review and skip drafts
• Add concurrency group per PR with cancel-in-progress: true so old runs are killed
• urlopen(timeout=120) prevents hangs (root cause of the 2m+ jobs before the 429 landed)
• Retry 3x on 429 with random(30,60) * attempt sleep; retry 5xx with 5 * attempt
• Fallback to openai/gpt-4o if primary is still rate-limited after retries
• Cap diff at 8000 chars (was 12000) with truncation note in the comment

Context

Addressing intermittent HTTP 429 from LiteLLM gateway on openai/gpt-5.3-codex. Previously the job would hang ~2.5min before the gateway rejected with 429; now it fails or falls back predictably.

Test plan

• Trigger via workflow_dispatch and verify retries log on 429
• Confirm superseded runs get cancelled when a new PR event fires
• Verify fallback path runs when primary exhausts retries

[github-actions]

@claude review