Abstention: add an entity-existence gate before retrieval (as in the tangleclaw-style row) #114

The tangleclaw-style reference row (thanks to Jason Vaughan, #87) abstains on 10/10 absent_entity questions with a cheap capitalized-entity check, and refuses only 3 of 69 answerable questions. Our vector row abstains on 6/10 absent entities and refuses 18/69 answerable ones.

Proposal: run an entity-existence check before retrieval: if a capitalized name in the question matches no note title, alias or entity in the index, return no answer. It needs no model and should lift absent_entity toward 1.00 while letting the reranker threshold be relaxed (see the paraphrase issue). Acceptance: abstain_correct on absent_entity 1.00, no new false abstains on fact or bridge.