refactor(nix): auto-load flake-modules + extract shared dotfiles-rebuild 🌳

- Add import-tree input; flake.nix now auto-loads every file under
  ./flake-modules so new hosts/features drop in without editing flake.nix.
- Extract the duplicated dotfiles-rebuild service, timer, and
  safe.directory wiring into nixos/modules/dotfiles-rebuild.nix, exposed
  via flake.nixosModules.dotfiles-rebuild.
- sunken-ship and phantom-ship now pull it in from their flake-modules;
  hostname-specific flakeRef is derived from config.networking.hostName.

nixos/flake.lock

@@ -154,6 +154,21 @@
         "type": "github"
       }
     },
+    "import-tree": {
+      "locked": {
+        "lastModified": 1773693634,
+        "narHash": "sha256-BtZ2dtkBdSUnFPPFc+n0kcMbgaTxzFNPv2iaO326Ffg=",
+        "owner": "vic",
+        "repo": "import-tree",
+        "rev": "c41e7d58045f9057880b0d85e1152d6a4430dbf1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "vic",
+        "repo": "import-tree",
+        "type": "github"
+      }
+    },
     "nix-darwin": {
       "inputs": {
         "nixpkgs": [
@@ -303,6 +318,7 @@
         "disko": "disko",
         "flake-parts": "flake-parts",
         "home-manager": "home-manager",
+        "import-tree": "import-tree",
         "nix-darwin": "nix-darwin",
         "nix-openclaw": "nix-openclaw",
         "nixos-wsl": "nixos-wsl",