fix(entity-gate): avoid false abstain on topic phrases + date fragments

[benjsmith]

Summary

• Stop entity_gate from false-abstaining when a query mixes a long topic phrase with abbreviated months / numeric date fragments (e.g. “Mar 4th, 2024”). Abbreviated months are stopwords; numeric date fragments are skipped; exact identity resolves before longest-first fuzzy suppression; duplicate n-grams that resolve to the same page collapse.
• graph.py rebuild now emits degraded, wiki_embeddings_reason, and degraded_reason when embeddings are configured but skipped or incomplete.
• Motivating case: kb_qa/36-style questions about career-development discussion meetings in a dated week; fix is general robustness, not eval-key hardcoding.

Test plan

• python3 -m unittest tests.test_entity_gate -v (24 tests OK, including test_topic_phrase_and_abbreviated_date_do_not_false_abstain)
• Spot-check retrieve on a topic+date query against repair-v18 wiki: target career-development page in seeds; n_hits > 0