feat(flake): enhance formatting capabilities with treefmt integration

Updated `flake.nix` to incorporate `treefmt-nix` for comprehensive file formatting across multiple types, including Nix, shell, Python, and more. Refined the formatter output and application setup to streamline usage, allowing users to format all files with a single command. This enhancement improves the development workflow and accessibility of formatting tools.

Author: kzndotsh

flake.nix

@@ -59,6 +59,8 @@
       inherit system;
       config.allowUnfree = true;
     };
+    # Eval the treefmt modules from ./treefmt.nix
+    treefmtEval = treefmt-nix.lib.evalModule pkgs ./treefmt.nix;
   in {
     nixosConfigurations = {
       default = nixpkgs.lib.nixosSystem {
@@ -119,18 +121,18 @@
       };
     };
 
-    # Formatter output for nix formatting
-    # Uses nixfmt (official Nix formatter, RFC-style is now default)
-    # Reference: https://github.com/NixOS/nixfmt
-    # Usage: nix fmt
-    formatter.${system} = pkgs.nixfmt;
+    # Formatter output for comprehensive formatting (all file types)
+    # Uses treefmt-nix to format: Nix, shell, Python, YAML, JS/TS/JSON/MD, etc.
+    # Reference: https://github.com/numtide/treefmt-nix
+    # Usage: nix fmt (formats all file types via treefmt)
+    formatter.${system} = treefmtEval.config.build.wrapper;
 
-    # treefmt configuration
+    # treefmt app (alternative way to run treefmt)
     # Reference: https://github.com/numtide/treefmt-nix
     # Usage: nix run .#treefmt
     apps.${system}.treefmt = {
       type = "app";
-      program = "${(treefmt-nix.lib.evalModule pkgs ./treefmt.nix).config.build.wrapper}/bin/treefmt";
+      program = "${treefmtEval.config.build.wrapper}/bin/treefmt";
     };
 
     # Development shell (minimal - use devshells for project-specific tools)
@@ -156,7 +158,7 @@
           echo "  nh update         - Update flake inputs"
           echo ""
           echo "Formatting:"
-          echo "  nix fmt            - Format Nix files (single formatter)"
+          echo "  nix fmt            - Format all files (via treefmt)"
           echo "  treefmt            - Format all files (multiple formatters)"
           echo "  nix run .#treefmt  - Run treefmt via flake"
           echo ""