arch: Implement PostgreSQL + pgvector Storage Adapter for Sub-Millisecond Semantic Search

[kavix]

🎯 Goal

Implement a production-grade PostgreSQL 16 + pgvector storage adapter (internal/db/pgvector.go) for indexing code ASTs, diffs, and repository memory embeddings.

🛠️ Implementation Steps

1. Add pgvector-go or standard SQL driver with vector(1536) HNSW index support.
2. Add semantic similarity search query for eko ai ask and eko ai bisect.