fix: resolve code quality issues, add CI, and improve maintainability

- Remove useless nixConfig block (cache.nixos.org is default)
- Remove manual container-use override; let blueprint auto-discover it
- Add nixosModules.default so README example works
- Fix default launcher: use correct parameterized flake URL
- Replace deprecated sha256 with hash in goose-cli fetchers
- Fix LIBCLANG_PATH to include /lib subdirectory for libclang.so
- Drop --release from goose-cli tests (faster, more debug info)
- Use builtins.toFile in flake-inputs to avoid ARG_MAX risk
- Add lib.warn when overlay has no packages for a system
- Add passthru.updateScript to goose-cli, container-use, skillsmcp
- Fix skillsmcp version to 0.2.0+unstable (pinned to commit, not tag)
- Replace with lib; with explicit references in all meta blocks
- Add update.py script for goose-cli (referenced in AGENTS.md)
- Expand .gitignore with result-* and .direnv/
- Add GitHub Actions CI workflow (nix flake check + build)

overlays/default.nix

@@ -2,5 +2,9 @@
   packages,
 }:
 final: _prev: {
-  millerson-nix-overlay = packages.${final.stdenv.hostPlatform.system} or { };
+  millerson-nix-overlay =
+    packages.${final.stdenv.hostPlatform.system}
+      or (final.lib.warn "millerson-overlay: no packages for system ${final.stdenv.hostPlatform.system}"
+        { }
+      );
 }