refactor(nix): make all-local a deployment mode, not a default

mara: enableAllLocalDefaults is more of a deployment mode via settings
set, less a default setting.

That reframing is the change. A mode asserts values; an option declares
what it is when nobody asks. Written as default = <flag>, every service
option had to name a switch it has no relationship to, and the answer to
what does all-local turn on was spread across five files.

Two modules now hold the two tiers - local-defaults.nix for the mode and
what it asserts directly, swarm-required-services.nix for the
services-live-here switch and the per-service enables under it. Each
service module keeps its own declaration and loses only the line about
when a deployment wants it.

mkDefault is the right precedence rather than a compromise: it beats an
option default and yields to any explicit definition, so the mode fills
in for an operator who has not spoken and never argues with one who has.
Evaluated config is unchanged in both directions.

nix/host-modules/default.nix

@@ -12,6 +12,7 @@
 {
   imports = [
     ./hyperhive.nix
+    ./local-defaults.nix
     ./hive-c0re
     ./hive-ci.nix
     ./hive-forge
@@ -27,5 +28,6 @@
     ./swarm-snapshot-store.nix
     ./swarm-wireguard.nix
     ./swarm.nix
+    ./swarm-required-services.nix
   ];
 }