Add remote LLM backend support

[rutvik-netizen]

Summary

• Add a configurable remote OpenAI-compatible LLM backend for embeddings, query expansion, and reranking
• Wire backend selection through env vars, YAML model config, CLI, SDK store creation, and MCP via createStore
• Document remote backend usage and add unit tests for remote embedding, expansion, rerank, and backend selection

Testing

• CI=true bun test test/llm.test.ts
• CI=true bun test test/sdk.test.ts -t "store.embed forwards batch limit options|CollectionConfig type is usable"

Notes

• bunx tsc -p tsconfig.build.json --noEmit still reports the existing unrelated typing issue: src/store.ts(2143,22): Property 'transaction' does not exist on type 'Database'.

[rutvik-netizen]

Closing this PR because it was opened against the wrong upstream. Re-opening against northowl-io/qmd instead.