feat: validate memory index links

[solomonneas]

Summary

• add built-in brigade doctor validation for MEMORY.md links to memory/cards/*.md
• fail doctor when the slim bootstrap memory index points at missing durable memory cards
• report verified card-link counts and memory card counts in doctor output

Verification

• .venv/bin/python -m pytest tests/test_doctor.py tests/test_managed.py tests/test_init.py -q - 39 passed
• .venv/bin/brigade doctor --target .
• .venv/bin/python -m pytest -q - 252 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