feat: add NixOS support, Go, and Ruby on Rails; remove Rust

- Add nixos/ directory with declarative flake.nix and home.nix for NixOS
- Make all setup scripts NixOS-aware with proper detection and early exit
- Update bashrc, zshrc, and aliases with Go, Ruby on Rails, and Nix profile paths
- Replace Rust/Cargo references in shell configs with rbenv/Ruby on Rails
- Update README with NixOS quick start and full documentation

scripts/00-system-prep.sh

@@ -3,6 +3,7 @@
 # 00-system-prep.sh
 # Updates system and sets up Flathub.
 # Relies on DISTRO and PACKAGE_MANAGER being set by the caller.
+# Supports NixOS, Fedora (dnf), and Debian/Ubuntu (apt).
 
 echo "--- Starting System Preparation ---"
 
@@ -11,23 +12,32 @@ if [ -z "$DISTRO" ] || [ -z "$PACKAGE_MANAGER" ]; then
     exit 1
 fi
 
-# Update system before installing packages
-echo "Updating system packages..."
-if [ "$PACKAGE_MANAGER" == "dnf" ]; then
-    sudo dnf update -y && sudo dnf upgrade -y
-elif [ "$PACKAGE_MANAGER" == "apt" ]; then
-    sudo apt update && sudo apt upgrade -y
-else
-    echo "WARNING: Unknown package manager '$PACKAGE_MANAGER'. Skipping system update."
+# ── NixOS path: system updates are declarative via flake ──
+if [ "$DISTRO" == "nixos" ]; then
+    echo "NixOS detected: system updates are managed declaratively via flake.nix."
+    echo "Run 'nixos-rebuild switch --flake ~/dotfiles/nixos#nixos' to apply changes."
+    echo "Skipping imperative system update."
 fi
 
-# Setup Flatpak
+# ── Traditional distro path ────────────────────────────────
+if [ "$DISTRO" != "nixos" ]; then
+    echo "Updating system packages..."
+    if [ "$PACKAGE_MANAGER" == "dnf" ]; then
+        sudo dnf update -y && sudo dnf upgrade -y
+    elif [ "$PACKAGE_MANAGER" == "apt" ]; then
+        sudo apt update && sudo apt upgrade -y
+    else
+        echo "WARNING: Unknown package manager '$PACKAGE_MANAGER'. Skipping system update."
+    fi
+fi
+
+# Setup Flatpak (works on NixOS too if enabled in flake.nix)
 echo "Setting up Flathub repository..."
 if command -v flatpak &> /dev/null; then
     flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
 else
     echo "WARNING: flatpak command not found. Skipping Flathub setup."
-    echo "Ensure Flatpak is installed via 01-package-install.sh or manually."
+    echo "Ensure Flatpak is installed via your package manager or flake.nix."
 fi
 
 echo "--- System Preparation Finished ---"