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>

lib/src/cli/argv_to_request.dart

@@ -27,7 +27,7 @@ import 'package:clide/src/ipc/schema_v1.dart';
 /// split. Match the IDs the dispatcher exposes directly. `tail` and
 /// `events` are handled by the IPC server itself (streaming / cursor-pull
 /// event reads, T-129 / T-223) rather than the dispatcher.
-const Set<String> _umbrellaCommands = {'status', 'tail', 'events', 'version', 'ping', 'capabilities'};
+const Set<String> _umbrellaCommands = {'status', 'tail', 'events', 'version', 'ping', 'capabilities', 'instance'};
 
 /// Sealed result of translating argv. Caller (the IPC server, or the
 /// C client wrapper in T-126) handles either branch.