expand ClideTestApp with IPC, extension, and theme tests

Six improvements to the testmode harness:

1. Shell passthrough tests use the resolved shell instead of
   hardcoded /bin/zsh.
2. Exit code reflects test results (non-zero on any failure).
3. JSON summary line for machine-readable parsing.
4. IPC round-trip smoke (ping, version, unknown-cmd, encode/decode).
5. Extension lifecycle smoke (register + activate files, diff,
   git, terminal; theme loading for all four bundled themes).
6. Test categories via CLIDE_TESTMODE dart-define (toolchain, ipc,
   extensions, all). Makefile exposes TESTMODE_CATEGORY variable.

Also switches main.dart from bool.fromEnvironment to
String.fromEnvironment so category values other than "true" work.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/main.dart

@@ -51,8 +51,8 @@ Future<void> main() async {
   final binding = WidgetsFlutterBinding.ensureInitialized();
 
   // Test mode: skip the full app, run the test harness instead.
-  const testMode = bool.fromEnvironment('CLIDE_TESTMODE');
-  if (testMode) {
+  const testMode = String.fromEnvironment('CLIDE_TESTMODE');
+  if (testMode.isNotEmpty) {
     runApp(const ClideTestApp());
     return;
   }