Add Forge agent guide

[im-sham]

Summary

• Add a root AGENTS.md for future agents landing in Forge.
• Capture incident-memory ownership, Proofhouse suite boundaries, pointer/ref data hygiene, implementation seams, MCP/CLI guardrails, and local verification commands.
• Address audit finding P3-007 for Forge without touching the open incident-index remediation PR.

Validation

• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m ruff check .
• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m pytest
• PYTHONPATH=. /Users/shamimrehman/Projects/USMI/forge/.venv/bin/python -m compileall forge_cli tests
• git diff --check

Notes

• Docs-only change.