Introduce a centralized `fetch-utils.ts` to handle retry logic with proxy fallback, SSE line buffering to prevent chunk-boundary corruption, and graceful shutdown via active reader tracking.
Key changes:
- Add `fetchWithRetry` for automatic direct-to-proxy failover.
- Implement `SSELineBuffer` to ensure reliable parsing of split SSE chunks.
- Add `createStreamBodyLimiter` to enforce payload limits on streaming requests.
- Refactor `ProxyPool` to decouple failure marking from rotation.
- Standardize CORS handling and environment variable configuration.
- Clean up documentation and remove obsolete skill files.
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