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

zshrc

@@ -8,6 +8,11 @@ plugins=(git)
 # Editor
 export EDITOR='nvim'
 
+# Nix profile
+if [ -e "$HOME/.nix-profile/etc/profile.d/nix.sh" ]; then
+  . "$HOME/.nix-profile/etc/profile.d/nix.sh"
+fi
+
 # Go
 export GOPATH=$HOME/go
 export PATH=$PATH:/usr/local/go/bin:$GOPATH/bin
@@ -15,14 +20,15 @@ export PATH=$PATH:/usr/local/go/bin:$GOPATH/bin
 # Local bin
 export PATH="$PATH:$HOME/.local/bin:$HOME/bin"
 
+# Ruby on Rails (rbenv)
+export PATH="$PATH:$HOME/.rbenv/bin"
+eval "$(rbenv init - zsh 2>/dev/null || true)"
+
 # Node.js and NVM configuration
 export NVM_DIR="$HOME/.nvm"
 [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"
 [ -s "$NVM_DIR/bash_completion" ] && \. "$NVM_DIR/bash_completion"
 
-# Rust/Cargo
-[ -f "$HOME/.cargo/env" ] && source "$HOME/.cargo/env"
-
 source $ZSH/oh-my-zsh.sh
 source $HOME/dotfiles/aliases.zsh
 
@@ -42,4 +48,6 @@ export CPLUS_INCLUDE_PATH=/usr/local/include
 [ -f ~/.fzf.zsh ] && source ~/.fzf.zsh
 
 # Oh My Posh prompt
-eval "$(oh-my-posh init zsh --config ~/.config/oh-my-posh/theme.omp.json)"
+if command -v oh-my-posh &>/dev/null; then
+  eval "$(oh-my-posh init zsh --config ~/.config/oh-my-posh/theme.omp.json)"
+fi