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/content/hotkeys.ts

@@ -23,7 +23,7 @@ export const HOTKEYS: [string, string][] = [
   [paste + '+V / /paste', 'paste text; /paste attaches clipboard image'],
   ['Tab', 'apply completion'],
   ['↑/↓', 'completions / queue edit / history'],
-  ['Ctrl+X', 'delete the queued message you’re editing (Esc cancels edit)'],
+  ['Ctrl+X', 'open live session switcher (deletes queued message while editing)'],
   [action + '+A/E', 'home / end of line'],
   [action + '+Z / ' + action + '+Y', 'undo / redo input edits'],
   [action + '+W', 'delete word'],