fix: use proper zsh/bash completion methods and correct zsh install instructions

Use GenZshCompletion (native zsh) instead of the old bash-wrapper method.
Use GenBashCompletionV2 for modern bash completions. Fix help text to show
proper fpath-based zsh installation instead of `source`.

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

opal-task/cmd/completion.go

@@ -20,9 +20,16 @@ Bash:
   $ echo 'source <(opal completion bash)' >> ~/.bashrc
 
 Zsh:
-  $ source <(opal completion zsh)
+  # Option 1: Write to a directory in your $fpath
-  # To load on startup, add to ~/.zshrc:
+  $ opal completion zsh > "${fpath[1]}/_opal"
-  $ echo 'source <(opal completion zsh)' >> ~/.zshrc
+  # 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
 
 Fish:
   $ opal completion fish | source
@@ -33,7 +40,7 @@ Fish:
 	Run: func(cmd *cobra.Command, args []string) {
 		switch args[0] {
 		case "bash":
-			rootCmd.GenBashCompletion(os.Stdout)
+			rootCmd.GenBashCompletionV2(os.Stdout, true)
 		case "zsh":
 			rootCmd.GenZshCompletion(os.Stdout)
 		case "fish":