fix(cli): address review feedback on omz generate plugin

- Turn %placeholders% into private markers when a template is read and
  strip the marker from every value, so text inserted from user input
  (e.g. -d '%name%') is never rescanned as a placeholder.
- When --enable is used for a name already in $plugins (a custom
  override of an enabled built-in), report it and reload instead of
  failing in _omz::plugin::enable.
- Make the completion note accurate whether or not a _<cmd> file was
  generated.
- Test the write-phase cleanup with a template set missing a later
  template, and test that placeholder-looking values come out literally.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

templates/generators/plugin/completion-note.zsh-template

@@ -3,5 +3,4 @@
 #
 # Oh My Zsh adds this directory to $fpath, so a file named `_%command%` next to
 # this one is picked up automatically as the completion for %command%.
-# There is nothing to do here.
 #