[codex] Add public demo proof artifacts

[hudsonaikins]

Summary

Adds committed, sanitized PermitGraph demo artifacts so developers, reviewers, and AI agents can inspect real scanner output without running private repositories.

Changes

• Adds tools/build_public_demo_artifacts.py to regenerate public fixture scan outputs.
• Adds public fixture scan artifacts for safe-agent, risky-ci-agent, and risky-mcp-agent.
• Adds a Fumadocs page explaining how to use and interpret the demo artifacts.
• Links public proof artifacts from README and llms.txt.
• Fixes summary output so policy-evaluation.json only appears when policy mode created it.
• Adds regression coverage for normal scans and policy scans.

Validation

• python3 tools/release_check.py
• 134 Python tests passed
• repository self-scan passed with Permit status: approved
• Python package build passed
• dashboard lint/build passed
• documentation site build passed

Notes

This PR does not deploy docs, publish packages, or merge to main. It only publishes the branch and opens this draft PR.