feat: check memory care freshness

[solomonneas]

Summary

• report memory-care scan freshness from memory/cards/decay/scan-latest.json
• warn when the last decay scan is older than 7 days or has a missing/unparseable scan_date
• fail doctor when opted-in memory-care state is corrupt or refresh-queue cards is not a list
• document the distinction between advisory missing decay state and failing corrupt decay state

Verification

• .venv/bin/python -m pytest tests/test_doctor.py -q -> 20 passed
• .venv/bin/python -m pytest tests/test_doctor.py tests/test_managed.py tests/test_init.py -q -> 43 passed
• .venv/bin/brigade doctor --target . -> passed, with advisory missing memory-care decay warning
• .venv/bin/brigade work doctor --target . -> passed
• .venv/bin/python -m pytest -q -> 256 passed
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json -> Clean, 64 files checked
• git diff --check -> passed