cli: generalize the experts cell-target resolver to cell_target.go

resolveExpertsCellTarget was experts-named but nothing in it is experts-specific: it maps any repo (ULID or owner/repo) to the entire-api cell hosting it, exactly what every future repo-scoped or fan-out command needs (code search is next, PR #1616). Rename the file and identifiers — resolveRepoCellTarget, cellCoreClient, newCellCoreClient, cellResolveTimeout — and keep experts as the documented example consumer. Mechanical rename; no behavior change.