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/extension/src/extension.dart

@@ -0,0 +1,96 @@
+import 'package:clide/extension/src/contribution.dart';
+import 'package:clide/kernel/src/clipboard.dart';
+import 'package:clide/kernel/src/commands/palette.dart';
+import 'package:clide/kernel/src/commands/registry.dart';
+import 'package:clide/kernel/src/dialog.dart';
+import 'package:clide/kernel/src/events/bus.dart';
+import 'package:clide/kernel/src/files.dart';
+import 'package:clide/kernel/src/focus.dart';
+import 'package:clide/kernel/src/i18n/i18n.dart';
+import 'package:clide/kernel/src/ipc/client.dart';
+import 'package:clide/kernel/src/log.dart';
+import 'package:clide/kernel/src/net.dart';
+import 'package:clide/kernel/src/notify.dart';
+import 'package:clide/kernel/src/os.dart';
+import 'package:clide/kernel/src/panels/arrangement.dart';
+import 'package:clide/kernel/src/panels/registry.dart';
+import 'package:clide/kernel/src/project.dart';
+import 'package:clide/kernel/src/secrets.dart';
+import 'package:clide/kernel/src/settings.dart';
+import 'package:clide/kernel/src/theme/controller.dart';
+import 'package:clide/kernel/src/tray.dart';
+
+/// One shipping unit. Built-in extensions compile in as Dart subclasses;
+/// third-party extensions run as Lua scripts wrapped by a `LuaExtension`
+/// adapter (Tier 6).
+abstract class ClideExtension {
+  String get id;
+  String get title;
+  String get version;
+
+  /// IDs of other extensions that must be activated before this one.
+  /// Missing deps → this extension is skipped at load with a warning.
+  List<String> get dependsOn => const [];
+
+  /// The atoms this extension contributes.
+  List<ContributionPoint> get contributions;
+
+  /// Called once after dependencies activate.
+  Future<void> activate(ClideExtensionContext ctx) async {}
+
+  /// Called when the extension is disabled or the app shuts down.
+  Future<void> deactivate() async {}
+}
+
+/// Handed to every [ClideExtension.activate]. Lists every kernel service
+/// an extension may reach. The extension manager constructs a concrete
+/// instance with refs; tests can pass fakes.
+///
+/// The interface deliberately lists services individually rather than
+/// exposing a `KernelServices` aggregate — doing so would create an
+/// import cycle between the kernel facade and this file.
+abstract class ClideExtensionContext {
+  String get id;
+
+  Logger get log;
+  EventBus get events;
+  SettingsStore get settings;
+  ThemeController get theme;
+  I18n get i18n;
+  PanelRegistry get panels;
+  LayoutArrangement get arrangement;
+  CommandRegistry get commands;
+  PaletteController get palette;
+  ClideClipboard get clipboard;
+  FileServices get files;
+  Notifications get notify;
+  DialogRouter get dialog;
+  TrayRegistry get tray;
+  SecretsVault get secrets;
+  OsBridge get os;
+  NetworkStatus get net;
+  FocusTracker get focus;
+  ProjectManager get project;
+  DaemonClient get ipc;
+}
+
+/// Sugar for i18n lookups scoped to this extension's namespace.
+extension ClideExtensionContextI18n on ClideExtensionContext {
+  /// `ctx.t('welcome.title', placeholder: 'clide')` →
+  /// `i18n.string('welcome.title', namespace: id, placeholder: 'clide')`.
+  String t(String key, {String? placeholder}) =>
+      i18n.string(key, namespace: id, placeholder: placeholder);
+
+  /// [t] with interpolation replacers.
+  String tr(
+    String key, {
+    String? placeholder,
+    List<I18nReplacer> replacers = const [],
+  }) =>
+      i18n.interpolated(
+        key,
+        namespace: id,
+        placeholder: placeholder,
+        replacers: replacers,
+      );
+}