Adds .github/policy/prompt.md and schema.json (the full security
review rubric — malicious code, privacy, deception, safety
circumvention, exfiltration; plus network-call and software-install
flags) and points scan-plugins at it via the policy-prompt input.
With ANTHROPIC_API_KEY now configured on the repo, scan-plugins runs
the actual policy review on changed external entries instead of
no-op'ing.
