feat: add Local Cursor proxy

[raoabinav]

Evaluator Summary

• Abi added leann cursor, a loopback-by-default OpenAI-compatible proxy that lets local coding assistants retrieve LEANN code context before forwarding requests to Ollama, LM Studio, or another local model server.
• Design choice: the proxy keeps the client protocol standard (/v1/models and /v1/chat/completions) while injecting retrieved snippets into the system prompt, so editors can use it without a custom integration.
• The implementation includes streaming and non-streaming passthrough, explicit CORS allowlists, request-size limits, JSON validation, configurable retrieval depth, and opt-out embedding recomputation.
• Quality bar: 10 targeted tests cover proxy validation, retrieval injection, streaming behavior, and local-server safety, plus lint, format, type, lockfile, and whitespace checks.

[raoabinav]

Parking this PR for now to keep the review queue focused on #347. I’ll revisit/reopen this work later as a smaller clean PR once the content-hash PR lands.