Ship a Rails healthcheck for registered cells

Fizzy implemented a `/hotcellz` endpoint that reports whether its cells are usable, and every application will need the same thing.

Ship a Rails healthcheck for registered cells.

Placeholder for a future conversation — design not settled.

The issue names no files, tests, or implementation entry points; it only references Fizzy’s `/hotcellz` endpoint and registered cells. Start by reviewing that reference and the project’s existing Rails integration, then clarify the healthcheck design and its completion criteria before implementation.

Written by the indexing model from the issue text.