Wire threat model for agent discoverability + rename to THREAT_MODEL.md

Adds AGENTS.md (## Security pointer) + SECURITY.md wiring the
conventional AGENTS.md -> SECURITY.md -> THREAT_MODEL.md chain, and
renames the PMC-merged draft-THREAT-MODEL.md to the canonical
THREAT_MODEL.md (no longer a draft; matches the discoverable
convention). No model content changes.

Generated-by: Claude Code (Claude Opus 4.8)