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/host.dart

@@ -0,0 +1,36 @@
+import 'dart:io';
+
+import 'package:clide/extension/src/manifest.dart';
+
+/// Scans the third-party extensions root for `manifest.yaml` files.
+///
+/// Built-ins are registered by the app at boot; this scanner handles
+/// installed third-party extensions. Tier 0 returns an empty list
+/// until the Lua adapter lands — every call is safe to make anyway.
+class ExtensionScanner {
+  const ExtensionScanner();
+
+  /// Typical install root: `~/.clide/extensions/<id>/manifest.yaml`.
+  /// Override for tests.
+  Directory defaultRoot() {
+    final home = Platform.environment['HOME'] ?? '/tmp';
+    return Directory('$home/.clide/extensions');
+  }
+
+  Future<List<ExtensionManifest>> discover({Directory? root}) async {
+    final dir = root ?? defaultRoot();
+    if (!await dir.exists()) return const [];
+    final out = <ExtensionManifest>[];
+    await for (final entity in dir.list()) {
+      if (entity is! Directory) continue;
+      final m = File('${entity.path}/manifest.yaml');
+      if (!await m.exists()) continue;
+      try {
+        out.add(await ExtensionManifest.fromFile(m));
+      } on FormatException catch (_) {
+        // skip malformed manifests; the extensions-ui will surface them
+      }
+    }
+    return out;
+  }
+}