Add optional OpenRouter provider for embeddings/search pipeline

[jamiequint]

Summary

• Added provider selection via QMD_LLM_PROVIDER with local as the default.
• Added an OpenRouterLLM backend for embeddings, query expansion, and reranking.
• Kept privacy-safe defaults: no remote calls unless explicitly opted in.
• Added a one-time remote-inference notice per process when OpenRouter mode is active.
• Added API key loading from:
  • QMD_OPENROUTER_API_KEY (preferred)
  • OPENROUTER_API_KEY
  • QMD_OPENROUTER_API_KEY_FILE (defaults to ~/.config/qmd/openrouter.key)
• Made store/CLI LLM calls provider-agnostic.
• Added char-based chunking fallback for remote providers (no local tokenizer dependency).
• Added OpenRouter-focused tests and concise README docs for setup and key storage.

Testing

~/.bun/bin/bun test src/llm.openrouter.test.ts
~/.bun/bin/bun test src/llm.test.ts --test-name-pattern "Default LlamaCpp Singleton|LlamaCpp.modelExists"
~/.bun/bin/bun src/qmd.ts --help