feat(cli): clide instances / instance verbs for instance discovery (T-247)

Closes the observability half of T-247: a way to find and identify running
clides. `clide instances` scans the runtime socket dir, probes each live
*.sock, and prints its identity (version/pid/workspace/socketPath) as jsonl;
dead sockets are skipped. `clide instance` reports the one you're connected
to. Combined with CLIDE_SOCK honoring (this same ticket), you can now list
instances and pin the CLI to a chosen one.

Server: a new `instance` dispatcher command (registered in buildDispatcher
with the live workspace/pid/socket) returns the identity map; added to the
argv translator's umbrella set so a bare `instance` token routes to it.
Client: a POSIX dir-scan in clide.c (Windows stub until it ships).

Tests: e2e `instances` lists the live test server with its identity; a
cc-free unit test covers the `instance` command shape.

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

test/daemon/instance_command_test.dart

@@ -0,0 +1,19 @@
+/// Tests for the `instance` command verb (T-247) — this clide's identity
+/// (version / pid / workspace / socket path), the per-instance metadata the
+/// `clide instances` CLI verb aggregates. Covers the dispatch mechanics
+/// independent of the C client.
+library;
+
+import 'package:clide/clide.dart';
+import 'package:clide/src/daemon/instance_command.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('instance resolves and returns version/pid/workspace/socketPath', () async {
+    final d = DaemonDispatcher();
+    registerInstanceCommand(d, version: '1.2.3', pid: 4242, workspace: '/repo', socketPath: '/run/user/1000/clide/abc.sock');
+    final r = await d.dispatch(IpcRequest(id: '1', cmd: 'instance', args: const {}));
+    expect(r.ok, isTrue);
+    expect(r.data, {'version': '1.2.3', 'pid': 4242, 'workspace': '/repo', 'socketPath': '/run/user/1000/clide/abc.sock'});
+  });
+}