feat(tui): add Delete All (D) to kill all screens and Claude processes

- Add killAllScreensAndClaude() function to useSessionManager
- Kills all tracked sessions, orphan claudeman screens, and Claude CLI processes
- Press 'D' (Shift+d) in sessions view to trigger
- Update HelpOverlay with new shortcut documentation

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

src/tui/components/HelpOverlay.tsx

@@ -33,6 +33,7 @@ const SHORTCUT_GROUPS: ShortcutGroup[] = [
       { key: 'Enter', description: 'View session in TUI' },
       { key: 'a', description: 'Attach to screen (full terminal)' },
       { key: 'd', description: 'Delete/kill selected session' },
+      { key: 'D (Shift+d)', description: 'Delete ALL screens & Claude processes' },
       { key: 'c', description: 'Switch to cases view' },
       { key: 'n', description: 'Quick-start new session' },
       { key: 'r', description: 'Refresh list' },