fix: pass through "auto" model identifier to Cursor API

[sxin0]

Summary

When clients send model: "auto" (e.g. Claude Code CLI), the proxy now forwards it directly to Cursor CLI instead of treating it as unspecified and falling back to the last-used or default model. Cursor natively supports "auto" as a valid model identifier.

Root Cause

In resolve-model.ts, "auto" was explicitly excluded from being treated as an explicit model:

```ts
// Before
const explicitModel = requested && requested !== "auto" ? requested : undefined;
```

This caused "auto" to be dropped and replaced by lastRequestedModelRef.current or config.defaultModel.

Fix

"auto" is now detected and returned immediately, bypassing the fallback logic. It is intentionally not recorded in lastRequestedModelRef to avoid polluting model memory for subsequent requests.

Closes #7

Made with Cursor

[anyrobert]

great jobs on this! lgtm!