feat: forward account rate limits to clients as session metadata

[ortwinvanderstappen]

What

account/rateLimits/updated is already handled: the snapshot is stored in
sessionState.rateLimits and rendered by /status via formatRateLimitLines.
But the handler returns null, so nothing reaches the client — the numbers are
only visible if the user types a command.

This forwards them as session metadata too, using the existing
createCodexSessionInfoUpdate helper.

Why

It makes usage indicators free to build. A client that wants to show how much of
the plan is left has no way to learn it from the stream today, so it has to spawn
its own codex app-server and poll account/rateLimits/read — a second Codex
process and a duplicate request per refresh, for data the adapter already
receives, parses and keeps.

Forwarding it removes both: no extra process, no extra requests, no polling
machinery, and the figures update exactly when Codex says they changed.

Nothing is removed. /status keeps working, since the state is still recorded.

Change

One case in src/CodexEventHandler.ts:

             case "account/rateLimits/updated":
                 this.handleRateLimitsUpdated(notification.params);
-                return null;
+                return this.createCodexSessionInfoUpdate({
+                    rateLimits: notification.params.rateLimits,
+                });

Clients see the snapshot under _meta.codex.rateLimits, the same _meta.codex
convention as the other codex-specific session metadata.

Tests

src/__tests__/CodexACPAgent/rate-limit-events.test.ts — one snapshot of what
the client receives, so the wire format is visible in review.

• npm run typecheck — clean
• npx vitest run — 440 passed, 28 skipped, nothing else affected