feat: Vector store integration

[initcron]

Summary

Implement VectorStore trait and integrations per architecture docs.

Tasks

• Implement VectorStore trait from architecture
• Add Qdrant integration (primary)
• Add Chroma integration (development)
• Implement embedding providers (OpenAI, Cohere, local)
• Add vector search benchmarks

Reference

See docs/architecture/rust-implementation.md for complete trait definitions.