feat(shells): zoxide cd replacement in all three shells + guarded system-wide oh-my-zsh source

dotfiles/zsh/.zshrc

@@ -1,5 +1,14 @@
 source /usr/share/cachyos-zsh-config/cachyos-config.zsh
 
+# oh-my-zsh (system-wide package; no-ops where not installed)
+if [ -f /usr/share/oh-my-zsh/oh-my-zsh.sh ]; then
+  export ZSH=/usr/share/oh-my-zsh
+  source "$ZSH/oh-my-zsh.sh"
+fi
+
+# zoxide replaces cd in interactive shells; plain paths still pass through
+command -v zoxide >/dev/null && eval "$(zoxide init zsh --cmd cd)"
+
 # To customize prompt, run `p10k configure` or edit ~/.p10k.zsh.
 [[ ! -f ~/.p10k.zsh ]] || source ~/.p10k.zsh