search: harden semantic and hybrid scoring

[totocaster]

Note

It's not 100% clear whether this is required, or not, since AI agents can reason about current scores quite well.

Summary

The README roadmap calls for semantic snippet tuning, sqlite-vec regression fixtures, and hybrid scoring tweaks. Track that search hardening work so we can schedule it.

Tasks

• Evaluate semantic snippet generation and tune scoring thresholds.
• Add sqlite-vec regression fixtures to guard against vector store regressions.
• Revisit hybrid scoring weights and expose configuration where appropriate.

Definition of Done

• Semantic search returns higher-quality previews with documented thresholds.
• sqlite-vec regressions are covered by automated tests.
• Hybrid search has validated weighting defaults and documentation updates.