feat: add pluggable recall quality filtering

[Grivn]

Summary

• add pluggable deterministic recall quality policies with strict, balanced, and exhaustive built-ins
• filter low-confidence normalized scores before results enter Agent context
• expand candidates before filtering and report per-source quality statistics
• default strict selection to all high-confidence rows, at most 4 medium rows, and at most 2 unknown-scale rows
• document and validate configurable thresholds, budgets, and policy registration

Verification

• pnpm run verify: 343 tests passed, 1 platform smoke test skipped
• isolated benchmark: answerable context reduced 48.3% and absent-query context reduced 60.4%
• measured tradeoff: Recall@10 changed from 100% to 85.4%, while nDCG@10 changed from 0.948 to 0.923

Rollback / tuning

Set recallQuality.policy to balanced-v1 or exhaustive-v1, or tune maxMediumResults and maxUnknownResults.