Build the ephemeral codeSplitter office preview from apps/agent-office/MOVE.md

Follow-up to #1384. MOVE.md lists the preview checks (hire, queue to PR on board, merge round, rebuild, worker identity #942) and the gaps to close first: PTY over WebSocket in AgentSandbox, container lifetime, per-worker cost cap, auth mapping. Price it first; throwaway repo and token; destroy after verifying. The cut-over itself will be a MANUAL RUN: issue.