feat(nix): zerotier overlay via clan inventory + mac ZT client 🕸️

Stage 4b of the clan migration. Declares a clan.inventory.instances.zerotier
instance with sunken-ship as controller and phantom-ship as peer (controller
is also listed as a peer so it joins its own network). Generates the network
ID, controller identity, and per-peer identities via `clan vars generate`;
all secrets are SOPS-encrypted to the user's age key and the per-machine
age keys.

- nixos/sops/ — clan-managed SOPS state (user + per-machine age keys).
- nixos/vars/ — shared + per-machine zerotier vars; *-identity-secret
  files are SOPS-encrypted, *.value files are plain public data.
- clan.core.networking.{targetHost,buildHost} = "danny@<host>" on both
  servers so `clan machines update` knows where to push and build.
- mac gets `zerotier-one` installed as a homebrew cask; authorization
  on the controller happens manually by node-ID in a follow-up step.

Known rough edges (to chase in later stages):
- zerotier-inventory-autoaccept.service races zerotierone.service on
  first activation (connection refused against the local API). Retrying
  the unit succeeds; clan upstream bug.
- Deployment must go through `clan machines update`, not plain
  nixos-rebuild, or the per-host SOPS age key isn't uploaded and
  zerotier-one can't decrypt its identity.

nixos/sops/secrets/phantom-ship-age.key/secret

@@ -0,0 +1,14 @@
+{
+	"data": "ENC[AES256_GCM,data:43IKkW3YpbpEtECD3kXV4zWF6hB39knoWwqy5BGCqvYWSPccKIwwLD3ctCy3SeH806AatvE8Bl2dvHFvP++xtvFtw5PaHdnenn8=,iv:j7ODs5O0rbwD0LWkkv9BEk6O9ySl+uhCiEVa+GkRE3k=,tag:Bk/PkQjOvul8pP7hoh2cwQ==,type:str]",
+	"sops": {
+		"age": [
+			{
+				"recipient": "age1g6y8gvcampqj5y3yzdajke2h5n7k6ckdg6a424cghy5325px7cmqjmmd28",
+				"enc": "-----BEGIN AGE ENCRYPTED FILE-----\nYWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBVaVlVQ3pvRmpLaVB3WWtl\nYjBIdDBJK0VKeS90eE5YeFhFRnBPak5YckFZCkl5RkVMV3JxL0pSVkM4cjhRaUE3\nK24vSWM0YnFWeXNjc3ZSWDRBb1ZDeWsKLS0tIENabmsxVUl0UGZzN1pncWswTVdM\nWDBVTVMrYzJHUklKSVVjYXBBM2RuajgKCvrGjfjujmqq2lsbNAb8d1xUhv+es2uX\nydcfnqbFRF4pjrku41iRaOolWrZHDvl+PnMslk8bclZG23UKYbSkbA==\n-----END AGE ENCRYPTED FILE-----\n"
+			}
+		],
+		"lastmodified": "2026-04-19T12:31:43Z",
+		"mac": "ENC[AES256_GCM,data:7/Z1Up1DZUgNMCuuBh2pnfTH3Ih6824yJqD1+w9clqgkSrFtKL6v5oo5EV4TF2FDJcrYQtbbAWQoEgJXfCKXfIYOPBIChfoQEG5N5XxNe57bklkipOMWJBm7448qBhLgy3yJQqAVFkQw6uHTuDrcngRFW5D3xHkCSilHC/xau9U=,iv:WL98Dcuxojg6BQ5tLOuhXYCfFHVXqpIBr680uriPXz0=,tag:FCl6wkBiLJUyMu1RnOqeIw==,type:str]",
+		"version": "3.12.2"
+	}
+}

nixos/sops/secrets/phantom-ship-age.key/users/danny

@@ -0,0 +1 @@
+../../../users/danny