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.
Phase 27 Plan 01 keeps admin additive and host-boundary focused:
the feature generates a small host policy module, a host-owned shell
component, and a router injection that mounts /admin plus
/admin/organizations/:org using normal Phoenix scopes,
RequireAdminAccess, and AdminScope.
This module contains zero references to other install features.