The FactRetriever's _fts_candidates passed the raw query string directly
to FTS5's MATCH operator. FTS5 defaults to AND-between-tokens, which
means any multi-word prose query like 'what happened with the deployment
rollback' required every single token to co-occur in a fact — dropping
recall to zero on the kind of queries agents actually issue via prefetch().
Fix: add _sanitize_fts_query() that:
- tokenizes the query and drops English stopwords
- strips FTS5 operator characters per token
- OR-joins the remaining content tokens as phrase literals
For pathological inputs (all stopwords, empty), falls back to the raw
query so the caller sees zero results instead of a SQL error.
This is a pure-retrieval-quality fix — the HRR + Jaccard reranking
stages still keep precision high. Ships with 10 tests covering the
sanitizer and retrieval integration.
