dissolve app/ into repo root (D-056)

Single Flutter package at the repo root. All code, tests, assets,
and platform directories moved from app/ to root. Package renamed
from clide_app to clide — all imports rewritten. Merged pubspec
combines core (ffi) and app (flutter, yaml, xterm) dependencies.
Makefile simplified: no APP_PRESENT conditionals, no cd, no daemon
lifecycle. 317 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/extensions_ui/src/extension.dart

@@ -0,0 +1,17 @@
+import 'package:clide/extension/extension.dart';
+
+/// Tier-0 stub. Real implementation lands in a later tier; the extension
+/// is registered so the extensions-ui surface can list it as "installed,
+/// not yet implemented" and its id is reserved.
+class ExtensionsUiExtension extends ClideExtension {
+  @override
+  String get id => 'builtin.extensions-ui';
+  @override
+  String get title => 'Extensions UI';
+  @override
+  String get version => '0.0.0-stub';
+  @override
+  List<String> get dependsOn => const [];
+  @override
+  List<ContributionPoint> get contributions => const [];
+}