Sprint 3: Improve first-run experience (onboarding UX)
- Add EmptyState welcome card with quick-start guidance (open repo, j/k nav, ? shortcuts)
- Add ? hint badge in StatusBar that opens keyboard shortcuts help
- Wrap AppHeader sync/search buttons with Tooltip components showing shortcuts
- Organize CommandPalette into Repository/Tools/App groups with shortcut hints
- Add missing d (describe) and a (abandon) shortcuts to KeyboardShortcutsHelp
- Fix shortcut key labels for consistency (⌘ K, ⌘ ,)