feat(policy): add proposal provenance, operations, and guidance fields to proto #1096

Description

Add additive proto fields for agent-authored policy proposals, operation-shaped draft chunks, and typed rejection guidance.

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.

The MVP keeps summary/status fields string-typed where possible, but proposal operations should reuse the existing PolicyMergeOperation shape behind incremental policy updates.

Definition of Done

• PolicyChunk includes source, intent_summary, human_summary, and validation_result.
• PolicyChunk can carry agent-authored PolicyMergeOperation operations for approval-time merge.
• RejectDraftChunkRequest includes optional guidance.
• Generated Rust/Python bindings compile.
• Existing clients continue to work when fields are empty.
• Proto comments make clear that intent_summary is agent-supplied and not independently verified.