Add handoff ingest observability

[solomonneas]

Summary

• add optional ingestor.last_run_log support to handoff source config
• warn on stale ingest logs, skipped malformed handoffs, warning summaries, and no-reply/no-update masking signals
• expose ingestor health in text and JSON output, with capped warning details for daily doctors

Verification

• .venv/bin/python -m pytest tests/test_handoff_cmd.py tests/test_doctor.py tests/test_work_cmd.py -q
• .venv/bin/python -m pytest -q
• git diff --check
• .venv/bin/brigade handoff doctor --target .
• .venv/bin/brigade work doctor --target .
• .venv/bin/brigade doctor --target .
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json