groomer freshness: capture repository-context empty searches in the baseline #1115

Follow-up from #1091 (PR #1114).

Problem

#1091 saves empty search_code queries from exploration tool calls only. The repository-context pass (src/lib/groomer/repository-context.ts) also runs keyword searches before analysis; when one returns nothing and the tool loop is disabled, the run has repo-wide negative evidence with no saved query. The freshness baseline therefore records scope global with no queries, and every default-branch commit stales the result (global_evidence_commit) — the exact churn #1091 removed for exploration searches.

Scope

• Capture repository-context queries that returned zero results into the same bounded groomedSearchCodeQueries baseline field (or a parallel one), with the same bounds (dedupe, trim, max count/length).
• Keep the conservative behaviour when it cannot be determined whether a query was empty.
• Extend freshness tests to cover a dispatcher-only run whose only negative evidence is a repository-context search.

Expected files

src/lib/groomer/repository-context.ts
src/lib/groomer/freshness.ts

Out of scope: any change to staleness reasons or the recheck mechanics from #1091.