feat: add TUI session orchestrator

Add a first-class active-session orchestrator for the Ink TUI:

- list, activate, close, and launch live process-local TUI sessions
- hydrate committed and in-flight output when switching sessions
- dispatch a new prompt session from the +new row with session-scoped model picks
- expose a clickable live-session count in the status chrome
- preserve stable row order while initially focusing the current session
- support mouse hit-testing for floating orchestrator overlays
- add backend and frontend regression coverage for the lifecycle and UI helpers

ui-tui/src/app/uiStore.ts

@@ -15,6 +15,7 @@ const buildUiState = (): UiState => ({
   detailsModeCommandOverride: false,
   indicatorStyle: DEFAULT_INDICATOR_STYLE,
   info: null,
+  liveSessionCount: 0,
   inlineDiffs: true,
   mouseTracking: MOUSE_TRACKING,
   pasteCollapseLines: 5,