feat: implement BL-049 embedder error tolerance with BM25 fallback

[tryweb]

Summary

• BL-049 implementation complete: embedder retry with exponential backoff + BM25 fallback
• All OpenSpec artifacts created and archived
• Unit tests: 181 pass (3 new embedder tests)

Changes

• Add retry config (embedding.retry.*) to types and config parsing
• Add embedWithRetry() / dimWithRetry() wrappers in embedder.ts
• Add embedder health state management (getEmbedderHealth, setEmbedderHealth)
• Add BM25 fallback logic in memory_search and recall hook
• Extend memory_stats with searchMode and embedderHealth fields

Verification

• openspec validate: passed
• Unit tests: 181 pass (bun test)
• New embedder tests: 3/3 pass

Changelog

• internal-only: Improved embedder error handling and automatic fallback to BM25 search when embedding service is unavailable.