fix: improve package quality and fix goose-cli build

- Add meta.platforms to all packages for proper platform detection
- Add passthru.category to mcp-gateway for consistency
- Fix meta style inconsistency in mcp-gateway (lib.licenses → licenses)
- Fix pythonImportsCheck in skillsmcp to actually validate the import
- Remove empty maintainers list from skillsmcp
- Add libclang to goose-cli nativeBuildInputs (fixes bindgen/llama-cpp-sys-2 build)
- Add treefmt.toml with nixfmt formatter configuration
- Add nixConfig.extra-substituters for binary cache support
- Delete broken goose-cli/update.py (referenced missing scripts/updater.py)
- Document nix-update usage in AGENTS.md for package updates
- Fix stale project structure diagram in README

packages/skillsmcp/package.nix

@@ -28,7 +28,7 @@ python3Packages.buildPythonApplication rec {
 
   # Disable all checks to avoid version issues
   doCheck = false;
-  pythonImportsCheck = [ ];
+  pythonImportsCheck = [ "skillsmcp" ];
 
   # Patch to accept fastmcp 2.x from nixpkgs
   postPatch = ''
@@ -43,6 +43,6 @@ python3Packages.buildPythonApplication rec {
     homepage = "https://github.com/aviddiviner/skillsmcp";
     license = licenses.mit;
     mainProgram = "skillsmcp";
-    maintainers = with maintainers; [ ];
+    platforms = platforms.all;
   };
 }