feat(query): add --explain score traces for hybrid retrieval

[vyalamar]

Summary

• add --explain option to qmd query for retrieval score attribution
• expose backend scores (FTS/vector), per-list RRF contributions, top-rank bonus, reranker score, and final blended score
• include explain payload in JSON output and concise explain section in CLI output
• add unit tests for RRF trace correctness and bonus thresholds
• document the flag in README and add changelog entry

Validation

• npm run build
• npx vitest run --reporter=verbose test/rrf-trace.test.ts test/store.helpers.unit.test.ts