remove dead welcome tiles; advertise only real shortcuts (T-383)

Clone-from-git and Start-a-Claude-session were inert onTap: () {}
stubs whose printed shortcuts were never registered — dead UI on the
first screen a new user sees. No advertised dead ends: the tiles are
removed until their flows exist. The tips card was also fiction
(four of six shortcuts unregistered, ⌘ glyphs for a ctrl-based
default keymap) — it now lists six bindings that exist in the shipped
default preset / contributed commands, and the Open-folder glyph
matches the real ctrl+o binding.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/builtin/welcome/widget_test.dart

@@ -46,6 +46,10 @@ void main() {
       expect(find.text('clide'), findsOneWidget);
       expect(find.text('IDE for Claude Code CLI'), findsOneWidget);
       expect(find.text('Open folder…'), findsOneWidget);
+      // T-383: no advertised dead ends — these tiles were inert no-ops with
+      // unregistered shortcuts; they return only with working flows.
+      expect(find.text('Clone from git…'), findsNothing);
+      expect(find.text('Start a Claude session'), findsNothing);
     });
 
     testWidgets('TIPS card renders when the viewport is tall enough', (tester) async {