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/sunken-ship-age.key/secret

@@ -0,0 +1,14 @@
+{
+	"data": "ENC[AES256_GCM,data:Mk4Vfs0PvKI4Ynwmz+8myrFtPW1swn9PdtQoeZw0xh9aCT+o6IWstAUypuCfwSgPYkj8PFPi2yq7ysTzglBkhrThV9Zto48U2dA=,iv:jL1WHTpN3mVNQJ/ltHBFd7zMtVtRmh9RIJAnh1SiGZc=,tag:zmRAQvcg6FW1+bEvZd8D6g==,type:str]",
+	"sops": {
+		"age": [
+			{
+				"recipient": "age1g6y8gvcampqj5y3yzdajke2h5n7k6ckdg6a424cghy5325px7cmqjmmd28",
+				"enc": "-----BEGIN AGE ENCRYPTED FILE-----\nYWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSB6NzV5c2FDVndUSWRnZndR\ndXI2bEY2VGRiRndNbjZscHdjL0N0eHUrV1hZCmJMRllSdjNLWS8rcnlYLy94VUcy\ndDlXeUptaGdwb2ZsMW1UZHJoeW5CZzgKLS0tIDBkeUozUDd2YWpIRTFlK3M3K2RH\naW9CMnc1ZXRmM0x4MDYwVHVLZnVpR0UKZSowubfXrUemRSFNYo8hxSaeV6/egOi6\nmtmxPICosAV5VRbf8c5Hn3XGNGfOGVwwox+GmLjzqfpVsM9f2Qm9IQ==\n-----END AGE ENCRYPTED FILE-----\n"
+			}
+		],
+		"lastmodified": "2026-04-19T12:31:44Z",
+		"mac": "ENC[AES256_GCM,data:SaRWT7Q7joTgG7+LBL2icBQ4k2SJdFfDcPzV3IsBIMgVFC4kQNbkVr0BlTM4mgtfH+IxE8PBQu1v/JFo6kf43njnF3mD/Yzr/EsLxwVmD9U1DTpW+mr1EBUVLfiGqnVrTj2DhMdatKB1g8jRwAlpIcsmrlnsHIKjuSj5HKRIi7Q=,iv:YVV3BMhfh1ThIiYwW4uHUmUKqkHUtCy0i0owiAngKyg=,tag:f4UaL5ZjEp3Gkd6LGiq+uw==,type:str]",
+		"version": "3.12.2"
+	}
+}

sops/secrets/sunken-ship-age.key/users/danny

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