cli: add multi-cell fan-out helpers (groupReposByCell, fanOutCells)

The data plane has no server-side cross-cell aggregator: a query over all of the caller's repos must be fanned out to each cell hosting them and merged client-side (the entire.io BFF's code-search pattern). PR #1616 inlines that orchestration into search_cmd.go; this extracts the generic layer so code search — and any later repo-set command — shares one implementation:

• groupReposByCell: repo index → per-cell groups, deterministic order.
• resolveCellBaseURLs: cluster-catalog join on ClusterSlug↔Cluster.Slug. The catalog exposes no cell field, so joining the cell name against Slug (as #1616 currently does) only works when the two coincide.
• fanOutCells: parallel per-cell calls under a per-cell timeout, one shared auth.CellClientFactory (subject resolved once, one identity token per jurisdiction), partial failures isolated per result slot. Merge semantics stay with the command.

Also documents the three cell-routing shapes in CLAUDE.md.