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/secrets.dart

@@ -0,0 +1,34 @@
+/// Tier-0 in-memory stub for the OS-keychain-backed vault.
+///
+/// Lands on `libsecret` (Linux) and macOS Keychain in a later tier.
+/// The async API already matches the eventual platform-channel shape,
+/// so consumers don't need to change when the real backend arrives.
+class SecretsVault {
+  final Map<String, String> _memory = {};
+
+  Future<void> write({
+    required String extensionId,
+    required String key,
+    required String value,
+  }) async {
+    _memory['$extensionId/$key'] = value;
+  }
+
+  Future<String?> read({
+    required String extensionId,
+    required String key,
+  }) async {
+    return _memory['$extensionId/$key'];
+  }
+
+  Future<void> delete({
+    required String extensionId,
+    required String key,
+  }) async {
+    _memory.remove('$extensionId/$key');
+  }
+
+  Future<void> deleteAll({required String extensionId}) async {
+    _memory.removeWhere((k, _) => k.startsWith('$extensionId/'));
+  }
+}