Agent Office inside codeSplitter: preview it ephemerally, then move for real #1384

Make Agent Office run inside codeSplitter, so the office (workers, task queue, PR board) is a codeSplitter feature instead of a separate Bun binary (apps/agent-office).

Why

• One place for PRs, merge waves, the Mervin merge engine and the workers that fix conflicts.
• The office already leans on codeSplitter: the dispatch hint (Agent Office queue: show codeSplitter's dispatch hint before hiring a worker on an issue #1187), the "hung in X" CiRunner annotation (Office PR board and codeSplitter CiRunner: show "hung in X" from the hung annotation #1218), and the Wait hold on busy branches (Agent Office queue: Wait holds the task until its busy branches merge #1356).

The catch

The PR agent that fixes things runs on the office being moved. Moving it for real would cut off the agent that could repair it. So do it in two steps.

Plan

1. Ephemeral first. Deploy the office inside codeSplitter as a separate preview (its own stage, destroyed after verifying; price it first, smallest resources). The current office keeps running and keeps its PR agent. Drive the preview with a throwaway queue and check: hiring a worker, the task queue, the PR board, merge/rebuild flows, worker identity (MANUAL RUN: MAKE AGENT OFFICE AGENT AGAIN #942).
2. List what has to move. Worker terminals and git worktrees need a real machine or sandbox (AgentSandbox), so work out what stays local and what becomes a codeSplitter service. Write that list on this issue before building.
3. Then for real. Cut over only when the preview has run a full merge round without the old office's help. Keep the old binary until then.

Done when

• A preview office inside codeSplitter takes a task, runs a worker to a PR, and shows it on the board.
• The cut-over steps and rollback are written down.

MANUAL RUN parts (the cut-over itself needs the owner) get their own MANUAL RUN: issues.