Publish MCP delivery handoffs on issue timelines (AXI-109)

[Codename-11]

Summary

• teach the native MCP handshake the code-delivery and human-facing comment workflow
• add explicit work-session tool guidance for semantic status updates and durable PR handoffs
• add a settings-driven DeliveryTimelinePolicy: Off, Recommend, Require on PR, or Auto on PR
• let workSessions.attachPullRequest atomically attach the PR, advance delivery state, create/watch/audit one issue comment, and return the resulting timeline action
• keep repeated PR attachment idempotent so automatic handoffs cannot duplicate
• expose the policy in Workspace settings and add database/browser coverage

Why

The durable comment requirement existed in dispatched AgentRun instructions, but ordinary Codex Desktop work sessions only received the generic MCP handshake. A session could therefore claim, heartbeat, commit, and attach a PR while leaving no human-readable explanation on the issue timeline.

Behavior

• RECOMMEND remains the safe default and returns comments.create as the next action with a suggested template
• REQUIRE_ON_PR rejects first-time attachment unless timelineUpdate.body is supplied
• AUTO_ON_PR synthesizes a concise agent-authored PR handoff when the caller does not provide a richer one
• OFF records delivery state without prompting or commenting
• MCP PR attachment now requires both WRITE_ISSUES and WRITE_COMMENTS

Mechanical heartbeats remain audit-only. Interim human-facing phase changes continue through comments.upsertStatus.

Validation

• pnpm lint — passes with existing repository warnings
• pnpm typecheck
• Prisma schema validation and migration deploy against isolated Postgres
• full Vitest gate — 1,316 passed, 1 intentional live skip
• focused work-session and MCP-profile suites — 16 passed
• fresh production build
• focused Workspace settings Playwright journey — passed
• Prettier and git diff --check