feat: add security enrichment

[solomonneas]

Summary

• add explicit post-scan security enrichment artifacts and CLI
• add offline local enrichment and opt-in MISP provider config path
• surface enrichment in review, doctor, and work doctor
• clean up README walls of text into command-focused sections

Verification

• .venv/bin/python -m pytest tests/test_security_cmd.py tests/test_doctor.py tests/test_work_cmd.py -q
• git diff --check
• live brigade security scan/enrich smoke against a temporary workspace
• .venv/bin/python -m pytest -q
• PYTHONPATH=$HOME/repos/content-guard/src python3 -m content_guard scan . --policy $HOME/repos/content-guard/policies/public-repo.json
• rg -n "AgentShield|AgentShield-style|agentshield|affaan" .