fix: dedupe merged code-search stats across mirror cells

Fanning out code search across mirror placements makes a mirrored repo return the same matches from its home and mirror cells. mergeSearchResults already deduped the result list but summed the stats, so a repo with 3 matches searched in two cells reported "6 matches across 4 files in 2 repos" for 3 unique results, and falsely claimed truncation ("Showing 3 of 6 matches").

Dedupe RepoStats by repo name (keep the representative max, not the sum) and subtract the duplicated portion from TotalMatches / TotalFiles / ReposSearched. The base stays peregrine's own per-cell totals, so per-cell truncation and zero-match repos are preserved; only the provable mirror duplicate is removed.

Addresses trail #794 finding 019f40a4-d27.