feat: fail oversized memory cards

[solomonneas]

Summary

• add built-in brigade doctor memory-card budget checks
• fail memory/cards/*.md files over 8000 bytes and warn on empty cards
• document that memory cards and bootstrap files are part of doctor readiness

Verification

• .venv/bin/python -m pytest tests/test_doctor.py tests/test_managed.py tests/test_init.py -q -> 41 passed
• .venv/bin/brigade doctor --target . -> passed, including 11 memory cards <= 8000 bytes
• .venv/bin/python -m pytest -q -> 254 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