feat: fail oversized bootstrap files

[solomonneas]

Summary

• add built-in brigade doctor hard byte-budget checks for known bootstrap files
• fail doctor when installed bootstrap files exceed hard limits, preventing truncated agent startup context
• document the remediation path: keep bootstrap files slim and move durable detail into memory/cards/

Verification

• .venv/bin/python -m pytest tests/test_doctor.py tests/test_managed.py tests/test_init.py -q - 37 passed
• .venv/bin/brigade doctor --target .
• .venv/bin/python -m pytest -q - 250 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