refactor(nix): move flake to repo root 🚚

clan-cli silently ignores the `?dir=` URL parameter when resolving a
flake source, so with the flake at nixos/flake.nix `clan machines
update` fails with "flake.nix does not exist". Move the flake tree up
so the repo root contains flake.nix, flake.lock, flake-modules/, lib/,
modules/, sops/, and vars/. Host-specific NixOS modules stay in
nixos/{hosts,home,fish.nix,neovim.nix,…}; flake-module paths updated
accordingly.

- dotfiles-rebuild flakeRef is now "${dotfilesDir}#<host>" (was
  "${dotfilesDir}/nixos#<host>").
- CLAUDE.md build commands + clan section updated. nixupdate fish alias
  updated. sunken-ship hostsfile comment updated.
- Existing /etc/dotfiles checkouts on the servers will pick up the new
  layout on the next `dotfiles-rebuild` timer tick; the rebuild service
  was pre-updated via rsync so its flakeRef matches before the pull.

Also includes 4b follow-through: zerotier identities are now live on
both servers (sunken-ship=d553a2de33 controller, phantom-ship=6c048abbdc
peer) and IPv6 ping across the ZT mesh works.

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"
+	}
+}