[FEATURE]: TUI API - select session by ID from plugins

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Plugins can spawn child sessions via the SDK (client.session.create + client.session.promptAsync), but there's no way to programmatically switch the TUI to a specific session afterward.

The gap: There is a POST /tui/open-sessions that opens the sessions dialog, but the user has to manually find and select the right session from the list.

Use case: Multi-agent orchestration. A manager agent dispatches child sessions (e.g. a planner, then a dev) via a plugin. After spawning, it needs to hand off the user's attention to that child session. Right now the best we can do is open the dialog and show a toast saying "look for this title".

Proposed: A POST /tui/select-session endpoint (or extending open-sessions) that accepts { sessionID: string } and switches the TUI to that session.

Happy to open a PR but your contributing docs call for alignment first.