feat: expose rate limits on usage updates

[s-cork]

Summary

• Attach stored Codex rate-limit snapshots to ACP usage_update events under _meta["_codex/rateLimits"].
• Emit a refreshed usage update when rate limits change and context usage is already known.
• Retain rate limits received before token usage and include them in the next usage update.
• Merge sparse rolling updates with known values, including updates which omit an ID or name.
• Preserve the existing usage update when no rate-limit snapshot is available.

Why

codex-acp already stores account rate-limit snapshots for /status, but ACP clients cannot consume the structured values. This exposes the same snapshots without changing the standard ACP usage fields or making clients parse markdown.

Rate-limit notifications may omit unchanged values. Merging them with the previous snapshot prevents a rolling update from erasing windows, credits, plan metadata, or spend-control state.

Review the extension shape and event timing first, then the sparse-update merge.

Testing

Rebased onto current main at 50f69e57ca761ccafd2ca29de7fb591068277516.

• npm run typecheck
• npm test: 458 passed, 28 skipped
• npm run build
• git diff --check origin/main..HEAD

Tests cover deferred emission when rate limits arrive before token usage, immediate refresh after context usage is known, multiple independent limits, and complete-to-sparse updates which omit the ID or name.

Closes #227

AI assisted with implementation, rebase conflict resolution, tests, and PR text.