feat(tui): add temporary new session tab

[kitlangton]

What

Represent the unsaved /new screen as a selected, temporary New session tab.

Closing that tab now restores the previously active session from selection history. If the user submits a prompt instead, the temporary tab disappears as the created session takes its place in the strip.

Before / After

Before

1. Start on First task with Second task also open.
2. Run leader+n; the TUI moves to Home, but no tab represents that screen.
3. Run leader+w; because Home has no active tab, Close selects the last tab in strip order (Second task) rather than the previously active tab (First task).

After

1 First task  2 Second task
                          leader+n
1 First task  2 Second task  3 New session
                          leader+w
1 First task  2 Second task
^ restored

How

• Derive a non-persisted New session display tab whenever the active route is Home.
• Keep the placeholder outside the persisted session-tab collection and plugin tab API.
• Render the placeholder as selected and inert except for Close.
• Resolve Home-close through session selection history, falling back to strip order only when no open history entry remains.
• Let the existing session-route effect replace the placeholder with the real session tab after creation.

Scope

This changes only the TUI session tab strip. It does not create a durable session before prompt submission or expose a synthetic session ID through the plugin API.

Testing

• bun run test test/context/session-tabs.test.tsx test/context/session-tabs-model.test.ts in packages/tui: 26 passed
• bun typecheck in packages/tui
• Full repository typecheck through the pre-push hook: 33 tasks passed
• Terminal Control end-to-end run against a hermetic V2 server with synthetic First task and Second task sessions

Demo

The recording shows leader+n opening the temporary tab and leader+w closing it back to the previously active tab.

new-session-tab.mp4