Keep a cloud channel's listing in step with its compute service #430

Follow-up to #216 and #428. A cloud channel's listing is set only by what this host itself does, so it drifts from the compute service.

• Inactivity stop: services/compute stops a container after 6 hours without activity (INACTIVITY in services/compute/index.ts). The host's stopped flag changes only on its own Start compute and Stop compute (apps/host/src/compute.ts), so that channel still lists as running. Its menu offers only Stop compute, and Start compute on a running record fails with the service's "already running" error. Records created before fix(app): show a stopped cloud channel and offer only the compute action that applies #428 also list as running until their next start or stop.
• Busy and name: busy and rename projections reach the listing over the workspace link, and a compute record has none. The listing shows a cloud channel idle under its created name until the channel's own metadata is read (docs/architecture.md, Cloud compute (beta)).

Remaining

• Reflect the service's actual running/stopped state, including an inactivity stop, in the record and listing, and offer the action that applies.
• Carry busy and the current name into the listing for cloud channels.
• Verify against the deployed service with a channel that the service stops for inactivity.

The inactivity stop doesn't checkpoint, so work since the last checkpoint is lost; that durability gap is #413. This issue only covers what the listing shows.

Tracked by #58.