Session-diverse find regression gate

[catoncat]

Parent

#86

What to build

Add an eval-backed session-diversity gate for find results. The goal is to prevent one noisy session or repeated transcript family from dominating top hits while preserving dense reading once an agent selects a session.

Acceptance criteria

• Acceptance or dogfood fixtures include at least one query where repeated hits from one session family should not fill the top results.
• The runner reports top-result diversity metrics for that case.
• Any ranking change is minimal and justified by a failing-before/passing-after eval.
• Dense within-session follow-up remains available through existing read commands or result metadata.
• Existing source-aware/session-level recall tests remain stable.

Blocked by

• Answer-facet dogfood gate v1 #88