fix(tui): show shells from session location

[kitlangton]

What

Fixes the session composer Shell tab showing “No shell commands” when a background shell is running from a session whose location differs from the TUI bootstrap directory.

Before / After

Before

The background shell starts successfully and appears in the transcript, but the Shell tab begins with the bootstrap location’s shell collection and renders empty:

After

The Shell tab asks the TUI data context for active shells belonging to the current session across hydrated locations:

How

• data.shell.listBySession(sessionID) scans the TUI’s live location shell collections and filters by the existing metadata.sessionID association.
• ShellTab consumes that session-first read model, then limits the view to running shells.
• Shell execution, output, termination, and lifecycle remain location-owned; the tab’s existing kill action still uses each active UI location.

Scope

This fixes live TUI behavior across every hydrated location. A reconnect where the session still owns a shell in an old location that has not been rehydrated would require a server-side session-scoped active-shell query; that API question is being discussed separately.

Testing

• bun typecheck in packages/tui: clean.
• Full repository typecheck via pre-push hook: 33 tasks passed.
• OpenCode Drive ran the same nested-session/background-shell script against unmodified v2 and this branch. The baseline reproduced “No shell commands”; the fix displayed sleep 30 and restored the kill action.