move nixpkgs-stable to private flake inputs

now with home-manager and nix-darwin tests, we don't want to increase
the number of dependencies a user has to override in their flake.lock.

dev/private/flake.lock

@@ -0,0 +1,48 @@
+{
+  "nodes": {
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1731842749,
+        "narHash": "sha256-aNc8irVBH7sM5cGDvqdOueg8S+fGakf0rEMRGfGwWZw=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "bf6132dc791dbdff8b6894c3a85eb27ad8255682",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "release-24.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs-stable": "nixpkgs-stable",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs-stable"
+        ]
+      },
+      "locked": {
+        "lastModified": 1730321837,
+        "narHash": "sha256-vK+a09qq19QNu2MlLcvN4qcRctJbqWkX7ahgPZ/+maI=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "746901bb8dba96d154b66492a29f5db0693dbfcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

dev/private/flake.nix

@@ -0,0 +1,9 @@
+{
+  description = "private inputs";
+  inputs.nixpkgs-stable.url = "github:NixOS/nixpkgs/release-24.05";
+
+  inputs.treefmt-nix.url = "github:numtide/treefmt-nix";
+  inputs.treefmt-nix.inputs.nixpkgs.follows = "nixpkgs-stable";
+
+  outputs = _: { };
+}