[rushd][Epic] rushd — server-first re-architecture (daemon + thin clients)

Start with the RFC in #5863 and the dependency status in this epic, then read the outstanding workstreams #5898–#5901. This is an architectural coordination issue rather than a self-contained change. Done means the listed daemon default, fallback, transparency, and Rush 6 cutover conditions are met.

Re-architect Rush from a CLI-first tool into a server-first system: a long-lived per-workspace
daemon (rushd) owns the engine and warm build state; thin, presentation-owning clients (CLI first;
MCP/web later) talk a presentation-free protocol. Delivered as new split packages merged
incrementally into main at 0.x, culminating in a Rush 6 cutover.

Design rationale & discussion: #5863 (RFC).

Hard dependencies

• Watch-mode overhaul (#5378) merged — provides the stateful IOperationGraph
  (setEnabledStates, scheduleIterationAsync, invalidateOperations, abortCurrentIterationAsync,
  closeRunnersAsync) and OperationGraphHooks (onIdle, onExecutionStatesUpdated).
• Rush Reporter Overhaul (#5858) — supplies the presentation-free event contract & wire adapter;
  its "daemon-aligned major default flip" is the Rush 6 cutover.

Workstreams (sub-issues)

• WS0 (#5895) — Upstream engine prerequisites (in rush-lib)
• WS1 (#5896) — Wire layer: protocol, transport & presentation
• WS2 (#5897) — Daemon host, routing & scheduling
• WS3 (#5898) — Daemon lifecycle, reload & warm-set
• WS4 (#5899) — CLI client & command surface
• WS5 (#5900) — Rush 6 cutover: testing, observability & migration
• WS6 (#5901) — Web UX & MCP bridge

Definition of done for the Epic

• rush/rushx default to the daemon with a permanent in-process fallback (--no-daemon/RUSH_DAEMON=0).
• The transparency bar (#5900) passes on the flipped bins.
• Shipped as Rush 6.0.0 in the same major as #5858 phase 6.