fix(host): recover open transcript watches after channel workers restart

[iamnbutler]

Fixes #153. Dogfooding #5.

Before: when a channel's worker restarted (for example ace kill then ace ask), the open chat stayed blank or stale while listings updated. Worker connections and peer GatewayClients ended watches silently; the app only re-watched when its own gateway reconnected.

After: the app's transcript replays in place on the same gateway connection and shows new output.

Watch lifetime

• The app's useTranscript owns each watch. Relays (worker or hosted Connection, the gateway, a peer GatewayClient) report how a watch ended and drop it; they never re-watch on their own.
• A failure frame after a watch's accepted reply means it ended: released, or its worker, hosted connection or peer gateway closed. Only watch requests keep relay entries.
• useTranscript resets, re-fetches info and replays. It ignores watches it released and stops after 3 quick ends with Try again. A rejected watch retries only when asked, or once when an offline channel becomes reachable.
• Delete: remove() keeps the worker lock and removes channel.json first; workers read the record only after taking the lock. A recovering watch can't start a worker over a channel being deleted.

Rolling compatibility: the channel Event union is unchanged; older clients drop a late failure frame, as they do today. No stored format changes.

Evidence: bun types, bun run ci, React Doctor 100/100 on changed files, and GitHub CI. The real transport smoke used workers and Anthropic claude-haiku-4-5.

Headless React verification: 32 of 32 checks passed using the built app in headless Chromium against the isolated source host. Covered:

• kill while selected, then in-place restore;
• a composer ask;
• the 3-end limit and Try again;
• release on channel switch;
• CLI and app-menu delete;
• host restart.

Duplicates were checked with durable entry IDs. Details are in #153.

Pending

• Native app validation.
• A physical two-host interruption.
• Offline → reachable retry with real peers.
• Unknown-chat rejection, which the UI can't reach, is transport-only.

Built in Ace

The implementation, smoke runs, commits and PR were done by an agent in the Ace channel channel-watch-recovery. Codex was used outside Ace to coordinate concurrent lanes and to independently review the code and this PR, because Ace can't run Codex as an external harness yet (#9). React was verified in headless Chromium through the shell because Ace has no browser automation surface (#50), and native capture is blocked while the macOS session is locked.