cover the cli-install env fallback and stale-GUI activation paths

Adds the two missing cases that left lib/kernel/src/cli_install.dart:74
(default-environment constructor branch) and the builtin extension's
stale-GUI activation warning uncovered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/kernel/cli_install_test.dart

@@ -128,6 +128,14 @@ void main() {
       );
       expect(i.bundledClientCandidates, ['/opt/clide/bundle/clide-cli']);
     });
+
+    test('falls back to the process environment when no env is passed', () {
+      // No env → uses Platform.environment; the in-bundle candidate still
+      // derives from the exe dir and the install dir from $HOME.
+      final i = CliInstaller(resolvedExecutable: '/opt/clide/bundle/clide');
+      expect(i.bundledClientCandidates, contains('/opt/clide/bundle/clide-cli'));
+      expect(i.installDir, endsWith('/.local/bin'));
+    });
   });
 
   group('expandedPath', () {