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/kernel/src/files.dart

@@ -0,0 +1,61 @@
+import 'dart:async';
+
+import 'package:clide/kernel/src/events/bus.dart';
+import 'package:clide/kernel/src/events/types.dart';
+import 'package:clide/kernel/src/panels/slot_id.dart';
+import 'package:flutter/foundation.dart';
+
+class FilesDropped extends ClideEvent {
+  const FilesDropped({required this.paths, required this.slot});
+
+  final List<String> paths;
+  final SlotId slot;
+
+  @override
+  String get subsystem => 'files';
+  @override
+  String get kind => 'dropped';
+  @override
+  Map<String, Object?> payload() => {
+        'paths': paths,
+        'slot': slot.value,
+      };
+}
+
+/// Tier-0 stub for file pickers and drop targets.
+///
+/// Flutter desktop has no native picker API without a dep; rather than
+/// add one now, pickOpen/pickSave/pickDirectory throw UnimplementedError
+/// and the drop target is a no-op until we wire it through the
+/// platform channel. This lets the rest of the kernel compile and makes
+/// the service surface real.
+class FileServices {
+  FileServices(this._events);
+  final EventBus _events;
+
+  Future<List<String>> pickOpen({
+    List<String> extensions = const [],
+    bool multiple = false,
+  }) async {
+    throw UnimplementedError('pickOpen — wired in a later tier');
+  }
+
+  Future<String?> pickSave({
+    String? defaultName,
+    List<String> extensions = const [],
+  }) async {
+    throw UnimplementedError('pickSave — wired in a later tier');
+  }
+
+  Future<String?> pickDirectory() async {
+    throw UnimplementedError('pickDirectory — wired in a later tier');
+  }
+
+  /// Invoked by the platform drop-target wiring when files land on a
+  /// slot. Emits a [FilesDropped] event; the slot-owning extension
+  /// subscribes.
+  @visibleForTesting
+  void notifyDropped({required List<String> paths, required SlotId slot}) {
+    _events.emit(FilesDropped(paths: paths, slot: slot));
+  }
+}