feat(policy): suggest L7 scoping for known REST hosts

[zredlined]

Description

Add a small MVP hint that flags agent-drafted L4 rules targeting known REST hosts and suggests protocol: rest for stronger method/path scoping.

Context

Parent: #1062
RFC artifact: https://github.com/NVIDIA/OpenShell/blob/feat/agent-driven-policy-management/rfc/0001-agent-driven-policy-management.md

This is part of the locked Agent-Driven Policy Management MVP. GitHub issues are the development source of truth; Linear is only a roadmap pointer.

This is an intentional bootstrap heuristic, not the long-term architecture. Future work should move toward observed L7 capability detection and computed consequence modeling.

Definition of Done

• Known REST hosts include at least api.github.com and Slack API host(s) used by the demo/pitch.
• Agent-authored L4 proposal for a known REST host gets a validation result hint to consider L7 REST scoping.
• Hint does not block proposal submission or approval.
• Tests cover known-REST and non-REST/fallback cases.