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

flake.nix

@@ -1,6 +1,10 @@
 {
   description = "Various packages for Nix";
 
+  nixConfig = {
+    extra-substituters = [ "https://cache.nixos.org" ];
+  };
+
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
     systems.url = "github:nix-systems/default";