Initial NixOS flake-based multi-host setup

Add a NixOS system configuration using flakes, supporting multiple hosts (desktop, laptop, my-machine) with shared and per-host settings. Includes Home Manager integration, user-level and system-level package management, and example hardware configuration placeholders. Provides a README with setup instructions and a .gitignore for common Nix and editor artifacts.

.gitignore

@@ -0,0 +1,13 @@
+# Nix build results
+result
+result-*
+
+# Temporary files
+*~
+*.swp
+*.swo
+.DS_Store
+
+# Hardware configuration (may contain sensitive info)
+# Uncomment if you don't want to commit hardware configs
+# hosts/*/hardware-configuration.nix