maint: Move treefmt to dev partition

Moves treefmt-nix input and configuration from the main flake to the
dev partition, since it's a development-only dependency. Also switches
pre-commit hooks from individual formatter hooks to the unified treefmt
hook for better consistency.

flake.lock

@@ -344,8 +344,7 @@
         "flake-parts": "flake-parts",
         "nix": "nix",
         "nix-cargo-integration": "nix-cargo-integration",
-        "nixpkgs": "nixpkgs",
-        "treefmt-nix": "treefmt-nix"
+        "nixpkgs": "nixpkgs"
       }
     },
     "rust-overlay": {
@@ -412,26 +411,6 @@
         "repo": "treefmt-nix",
         "type": "github"
       }
-    },
-    "treefmt-nix": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1762938485,
-        "narHash": "sha256-AlEObg0syDl+Spi4LsZIBrjw+snSVU4T8MOeuZJUJjM=",
-        "owner": "numtide",
-        "repo": "treefmt-nix",
-        "rev": "5b4ee75aeefd1e2d5a1cc43cf6ba65eba75e83e4",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "treefmt-nix",
-        "type": "github"
-      }
     }
   },
   "root": "root",