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>

CHANGELOG.md

@@ -18,6 +18,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **`clide instances` / `clide instance` CLI verbs.** `instances` lists every
+  live clide on the machine with its version, pid, workspace, and socket path
+  (probing the runtime dir); `instance` reports the one you're connected to — so
+  you can find and target a specific instance via `CLIDE_SOCK`. (T-247)
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