docs: simplify zsh completion instructions to eval in .zshrc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/completion.go

@@ -20,16 +20,8 @@ Bash:
   $ echo 'source <(opal completion bash)' >> ~/.bashrc
 
 Zsh:
-  # Option 1: Write to a directory in your $fpath
-  $ opal completion zsh > "${fpath[1]}/_opal"
-  # Then restart your shell or run: compinit
-
-  # Option 2: Write to a custom directory and add it to fpath
-  $ mkdir -p ~/.zsh/completions
-  $ opal completion zsh > ~/.zsh/completions/_opal
-  # Add to ~/.zshrc (before compinit):
-  #   fpath=(~/.zsh/completions $fpath)
-  #   autoload -Uz compinit && compinit
+  # Add to ~/.zshrc:
+  eval "$(opal completion zsh)"
 
 Fish:
   $ opal completion fish | source