make the clide command surface self-describing

Parity guarantees a verb exists for every UI action, but a verb is
unreachable if nothing advertises it. Add `clide capabilities` — it
reflects the live dispatcher registry to JSON (subsystem, verb, arg
schema) so the surface is discoverable and can't drift from what
dispatches. A thin /clide skill points Claude at it rather than
hard-coding a verb list, so new panels become reachable the moment
they register.

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

test/daemon/dispatcher_test.dart

@@ -2,6 +2,7 @@
 library;
 
 import 'package:clide/clide.dart';
+import 'package:clide/src/ipc/command_schema.dart';
 import 'package:test/test.dart';
 
 IpcRequest _req(String cmd, {String id = '1', Map<String, Object?> args = const {}}) {
@@ -51,7 +52,44 @@ void main() {
       expect(d.isEmpty, isFalse);
     });
 
-    test('clear removes user handlers but keeps ping + version', () async {
+    test('capabilities reflects the live registry with schemas (T-248)', () async {
+      final d = DaemonDispatcher();
+      d.register('echo', (req) async => IpcResponse.ok(id: req.id, data: const {}));
+      d.register(
+        'pane.resize',
+        (req) async => IpcResponse.ok(id: req.id, data: const {}),
+        schema: const CommandSchema(
+          positional: ['id', 'cols'],
+          args: {
+            'id': ArgSpec(required: true),
+            'cols': ArgSpec(type: ArgType.number, min: 1),
+          },
+        ),
+      );
+
+      final r = await d.dispatch(_req('capabilities'));
+      expect(r.ok, isTrue);
+      final commands = r.data['commands'] as Map<String, Object?>;
+      // Built-ins + the two just registered are all discoverable.
+      expect(commands.keys, containsAll(['ping', 'version', 'capabilities', 'echo', 'pane.resize']));
+
+      // Subsystem/verb split.
+      final resize = commands['pane.resize'] as Map<String, Object?>;
+      expect(resize['subsystem'], 'pane');
+      expect(resize['verb'], 'resize');
+      expect(resize['positional'], ['id', 'cols']);
+      final args = resize['args'] as Map<String, Object?>;
+      expect((args['id'] as Map)['required'], true);
+      expect((args['cols'] as Map)['type'], 'number');
+      expect((args['cols'] as Map)['min'], 1);
+
+      // A schema-less command carries no positional/args keys.
+      final echo = commands['echo'] as Map<String, Object?>;
+      expect(echo['subsystem'], '');
+      expect(echo.containsKey('args'), isFalse);
+    });
+
+    test('clear removes user handlers but keeps the built-ins', () async {
       final d = DaemonDispatcher();
       d.register('extra', (req) async => IpcResponse.ok(id: req.id, data: const {}));
       expect(d.isEmpty, isFalse);