feat(tui): add delete session from start screen

- Press 'd' or 'x' to delete/kill selected session
- Auto-adjust selection index when sessions are removed
- Update footer controls with delete shortcut
- Update help overlay with delete shortcut

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/tui/App.tsx

@@ -204,6 +204,11 @@ export function App(): React.ReactElement {
     return <HelpOverlay onClose={() => setShowHelp(false)} />;
   }
 
+  // Handle delete session from start screen
+  const handleDeleteSession = useCallback((session: ScreenSession) => {
+    killSession(session.sessionId);
+  }, [killSession]);
+
   // Render start screen
   if (viewMode === 'start') {
     return (
@@ -211,6 +216,7 @@ export function App(): React.ReactElement {
         sessions={sessions}
         onSelectSession={handleSelectSession}
         onAttachSession={handleAttachSession}
+        onDeleteSession={handleDeleteSession}
         onCreateSession={handleCreateSession}
         onRefresh={refreshSessions}
         onExit={exit}