test: cover lib/extension/ to 100% (T-89)

Adds tests for the previously-uncovered ClideExtensionContext sugar
(publish/subscribe/t/tr) and the default no-op ClideExtension
lifecycle hooks, driving the real context the ExtensionManager builds,
plus ExtensionScanner.defaultRoot + the no-arg discover() fallback.
lib/extension/ goes 74.1% -> 100%; total 95.06% -> 95.23%. Floor
unchanged (no integer crossing).

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

test/extension/src/host_test.dart

@@ -52,5 +52,19 @@ void main() {
       final out = await const ExtensionScanner().discover(root: root);
       expect(out.map((m) => m.id).toList(), ['ext.ok']);
     });
+
+    test('defaultRoot points at ~/.clide/extensions', () {
+      final scanner = ExtensionScanner();
+      final home = Platform.environment['HOME'] ?? '/tmp';
+      expect(scanner.defaultRoot().path, '$home/.clide/extensions');
+    });
+
+    test('discover() with no root falls back to defaultRoot', () async {
+      // Exercises the `root ?? defaultRoot()` fallback. The install
+      // root rarely exists in CI, so this returns a (possibly empty)
+      // list rather than throwing.
+      final out = await const ExtensionScanner().discover();
+      expect(out, isA<List<ExtensionManifest>>());
+    });
   });
 }