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>

linux/flutter/generated_plugin_registrant.h

@@ -0,0 +1,15 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#ifndef GENERATED_PLUGIN_REGISTRANT_
+#define GENERATED_PLUGIN_REGISTRANT_
+
+#include <flutter_linux/flutter_linux.h>
+
+// Registers Flutter plugins.
+void fl_register_plugins(FlPluginRegistry* registry);
+
+#endif  // GENERATED_PLUGIN_REGISTRANT_