fix: fan out code search across mirror placements, not just home cell

groupReposByCell only read the top-level Cell/Jurisdiction/ID from RepoIndexEntry, which represents the repo's home placement. Mirror placements in other regions (stored in the Placements array with their own Cell, Jurisdiction, and placement-specific repo ID) were ignored, so code search only hit the home region.

Now when Placements is non-empty, each placement is added to its own cell group with the correct per-placement repo ID — matching the BFF's fan-out behavior. resolveCellBaseURLs gains a jurisdiction-based fallback for the cluster catalog join, since placement-derived groups lack a cluster slug.

Closes ENT-998