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/focus.dart

@@ -0,0 +1,28 @@
+import 'package:clide/kernel/src/panels/slot_id.dart';
+import 'package:flutter/foundation.dart';
+
+/// Tracks the currently focused contribution (tab id + slot). Backs
+/// `clide active`; extensions that need "which tab does the user care
+/// about right now?" read from here instead of poking Flutter's
+/// FocusScope directly.
+class FocusTracker extends ChangeNotifier {
+  SlotId? _slot;
+  String? _contributionId;
+
+  SlotId? get activeSlot => _slot;
+  String? get activeContributionId => _contributionId;
+
+  void setActive({required SlotId slot, required String contributionId}) {
+    if (_slot == slot && _contributionId == contributionId) return;
+    _slot = slot;
+    _contributionId = contributionId;
+    notifyListeners();
+  }
+
+  void clear() {
+    if (_slot == null && _contributionId == null) return;
+    _slot = null;
+    _contributionId = null;
+    notifyListeners();
+  }
+}