Sigra.Install.Feature implementation for the admin feature.
Owns every template under priv/templates/sigra.install/admin/.
The admin feature is enabled by default and omitted with
--no-admin.
The feature stays additive and host-boundary focused: it generates a
persisted, host-owned platform-admin grant seam and lifecycle tasks, a
policy with allow/deny tests, a host-owned shell component, and router
injections that mount /admin plus /admin/organizations/:org using
normal Phoenix scopes, RequireAdminAccess, and AdminScope.
This module contains zero references to other install features.