feat(agent): add proposal -> review -> commit workflow for AI edits

[PeterGuy326]

Summary

• Implement a proposal state machine: pending_review -> approved/denied/expired -> committed
• AI agents create proposals without mutating documents; human reviewers approve/deny
• Commit rechecks document permission and base revision at execution time
• Expiration, idempotency, and full audit correlation across the workflow lifecycle

Dependencies

• feat(api): add an active-room-aware content mutation and restore gateway #4 (active-room mutation gateway) for executing approved proposals
• feat(audit): record consequential document and permission actions #5 (audit trail) for recording proposal/reviewer/execution correlation

Test plan

• Verify creating a proposal does not mutate the document
• Verify approval/denial state transitions are enforced
• Verify stale/conflicting proposals fail safely at commit time
• Verify denied/expired proposals cannot execute
• Verify commit is idempotent and produces a version record
• Verify revoked permission at commit time is caught
• Run npx vitest run src/__tests__/lib/api-v1-proposals.test.ts

Closes #9