Add HNSW-based vector search with configurable embedding providers
(OpenAI, Ollama) and automatic FTS5 fallback when no provider is
configured. Background pipeline embeds messages asynchronously on
ingest, stores vectors in a pure-Go HNSW index, and retries on
failure with exponential backoff. The search_messages MCP tool now
supports search_mode (auto/semantic/fulltext) and returns ranked
results with similarity scores. All existing tests continue to pass,
CGO_ENABLED=0 cross-compilation verified.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
