Fix PR watcher stale failed-job handling

[cbusillo]

Summary

• Ignore failed jobs from completed historical workflow runs when deciding current PR CI actions
• Preserve early diagnosis for failed jobs in active workflow runs
• Add regression coverage for stale completed-run failures and active-run failures

Verification

• python3 -m py_compile .codex/skills/babysit-pr/scripts/gh_pr_watch.py .codex/skills/babysit-pr/scripts/test_gh_pr_watch.py
• direct import/assert smoke for stale and active failed-job behavior
• ./build-fast.sh

Notes

• Follow-up from Codex review on PR Merge upstream 0.6.97 into local overlay #12.
• Local pytest is unavailable in this environment (python3 -m pytest reports no module named pytest).