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/git/src/extension.dart

@@ -0,0 +1,33 @@
+import 'package:clide/builtin/git/src/git_panel_view.dart';
+import 'package:clide/builtin/git/src/git_status_item.dart';
+import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+
+class GitExtension extends ClideExtension {
+  @override
+  String get id => 'builtin.git';
+  @override
+  String get title => 'Git';
+  @override
+  String get version => '0.1.0';
+  @override
+  List<String> get dependsOn => const ['builtin.diff'];
+
+  @override
+  List<ContributionPoint> get contributions => [
+        TabContribution(
+          id: 'git.panel',
+          slot: Slots.sidebar,
+          title: 'Git',
+          titleKey: 'tab.title',
+          i18nNamespace: id,
+          priority: -80,
+          build: (_) => const GitPanelView(),
+        ),
+        StatusItemContribution(
+          id: 'git.branch',
+          priority: 10,
+          build: (_) => const GitStatusItem(),
+        ),
+      ];
+}