The facts mechanism formatted the roster in Go in the control plane — one
formatter per fact, in the consumer's own configuration language. ADR 0120
makes a fact a path and a template: the mesh owns the data, the module owns
the format, and the control plane holds no format at all.
to-be 29 (operator accounts + what lives under a home) is rewritten to ride
it: the ssh files become roster templates, the whole ~/.ssh is owned with a
found/owned boundary that cannot lock the operator out, keys are mesh-owned
through an SSH CA (existing keys adopted not regenerated, the operator's
personal key signed not minted), and the ssh-agent is a user-scoped service.
