feat: RAG pipeline implementation

[initcron]

Summary

Implement RAG pipeline per architecture docs.

Tasks

• Implement RAGPipeline from rust-implementation.md
• Add chunking strategies (fixed, semantic, markdown)
• Implement hybrid search (semantic + keyword)
• Add reranking support
• Build context manager with token optimization

Performance Targets

• Vector search (top-5): <50ms
• RAG with reranking: <250ms
• Full context building: <400ms

Reference

See docs/architecture/ for detailed designs.