fix: move unsafeRoutes to module default, genesis overrides to []

Per review feedback:
- nebula module now defaults unsafeRoutes to [{route=10.42.0.0/16 via=10.157.0.2}]
  so all regular nodes get home LAN routing automatically
- genesis overrides unsafeRoutes=[] since it IS the routing node (avoids loop)
- exodus and all k3s nodes (hosea, isaiah, jeremiah, zeke) simplified to
  nebula.enable = true only, relying on the new default

hosts/unstable/isaiah/default.nix

@@ -42,15 +42,7 @@
       vip = metadata.hosts.${config.networking.hostName}.ip;
       priority = 255;
     };
-    nebula = {
-      enable = true;
-      unsafeRoutes = [
-        {
-          route = "10.42.0.0/16";
-          via = "10.157.0.2"; # genesis
-        }
-      ];
-    };
+    nebula.enable = true;
     podman.enable = true;
     tailscale = {
       enable = true;