feat(policy): expose sandbox-local policy.local API

[zredlined]

Description

Expose a sandbox-local policy.local HTTP API that lets agents inspect current policy, read recent denials, read their own sandbox-local activity logs, run advisory validation, submit policy proposals, and poll proposal status.

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 replaces the earlier openshell-policy CLI plan. The MVP should make the local API load-bearing; MCP/CLI wrappers can be added later if they prove useful.

API shape

Initial endpoints:

• GET /v1/policy/current returns the current effective policy for this sandbox.
• GET /v1/denials?last=N returns recent structured denial events for this sandbox.
• GET /v1/logs?last=N returns sandbox-local activity/log events the agent is allowed to inspect.
• POST /v1/prove runs an advisory static/prover check for a candidate operation set.
• POST /v1/proposals accepts a JSON body with PolicyMergeOperation-shaped operations, intent/summary metadata, and optional denial/log evidence refs; the supervisor/local service bundles this into the gateway gRPC proposal path for human approval.
• GET /v1/proposals/{id} returns proposal status, validation result, and rejection guidance.

Definition of Done

• policy.local resolves from inside the sandbox without requiring policy allow rules.
• API is sandbox-scoped and cannot read or mutate other sandboxes.
• Current policy endpoint reads the resolved effective policy.
• Denials endpoint reads structured OCSF JSONL events.
• Logs endpoint exposes only sandbox-local activity/log context safe for the in-sandbox agent to inspect.
• Proposal submit accepts PolicyMergeOperation-shaped JSON payloads instead of CLI flag strings.
• Supervisor/local service bundles proposal JSON into gateway gRPC draft chunks.
• Sandbox-origin submit stores draft chunks for approval and never applies policy directly.
• Submit returns proposal/chunk IDs for watch/status.
• Status endpoint reads GetDraftPolicy state and surfaces rejection guidance.
• MCP is explicitly not required for MVP.