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/mcp-gateway/package.nix

@@ -27,12 +27,15 @@ rustPlatform.buildRustPackage rec {
 
   doCheck = false;
 
+  passthru.category = "MCP Servers";
+
   meta = with lib; {
     description = "Universal Model Context Protocol gateway that sits between AI client and MCP tools/servers";
     homepage = "https://github.com/MikkoParkkola/mcp-gateway";
     changelog = "https://github.com/MikkoParkkola/mcp-gateway/releases/tag/v${version}";
-    license = lib.licenses.mit;
-    sourceProvenance = with lib.sourceTypes; [ fromSource ];
+    license = licenses.mit;
+    sourceProvenance = with sourceTypes; [ fromSource ];
     mainProgram = "mcp-gateway";
+    platforms = platforms.all;
   };
 }