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

bashrc

@@ -10,6 +10,11 @@ DEFAULT_USER="$USER"
 # ── 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
+
 # ── PATH additions ─────────────────────────────────────
 export GOPATH="$HOME/go"
 export PATH="$PATH:$GOPATH/bin:/usr/local/go/bin"
@@ -17,14 +22,18 @@ export PATH="$PATH:$HOME/.local/bin"
 export PATH="$PATH:$HOME/.pulumi/bin"
 export CPLUS_INCLUDE_PATH=/usr/local/include
 
+# ── Go ──────────────────────────────────────────────────
+# Go is available via Nix profile, home-manager, or /usr/local/go
+
+# ── Ruby on Rails (rbenv) ──────────────────────────────
+export PATH="$PATH:$HOME/.rbenv/bin"
+eval "$(rbenv init - bash 2>/dev/null || true)"
+
 # ── NVM ────────────────────────────────────────────────
 export NVM_DIR="$HOME/.nvm"
 [ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"
 [ -s "$NVM_DIR/bash_completion" ] && . "$NVM_DIR/bash_completion"
 
-# ── Rust/Cargo ─────────────────────────────────────────
-[ -f "$HOME/.cargo/env" ] && . "$HOME/.cargo/env"
-
 # ── SSH agent ──────────────────────────────────────────
 if [ -z "$SSH_AGENT_PID" ] || ! kill -0 "$SSH_AGENT_PID" 2>/dev/null; then
   eval "$(ssh-agent -s)"