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>

lib/tests/generate-plugin.test.zsh

@@ -75,6 +75,12 @@ else
   pass "foo has no unfilled placeholders"
 fi
 
+## Values that look like placeholders are written literally
+
+omz generate plugin lit -d '%compgen% and %name% stay' -c lit --yes >/dev/null 2>&1
+assert_contains "$ZSH_CUSTOM/plugins/lit/README.md" '%compgen% and %name% stay'
+assert_contains "$ZSH_CUSTOM/plugins/lit/lit.plugin.zsh" '%compgen% and %name% stay'
+
 ## A plugin with no command
 
 if omz generate plugin bar --yes >/dev/null 2>&1; then
@@ -151,6 +157,25 @@ else
 fi
 [[ -e "$ZSH_CUSTOM/plugins/quux" ]] && fail "quux was created by a failed run"
 
+## A failure after the first file is written removes everything it created
+
+broken="$(mktemp -d)"
+mkdir -p "$broken/templates/generators/plugin" "$broken/plugins"
+cp "$ZSH/templates/generators/plugin/plugin.zsh-template" \
+   "$ZSH/templates/generators/plugin/completion-note.zsh-template" "$broken/templates/generators/plugin/"
+# README.md-template is missing, so the plugin file is written and then the README fails
+if ( ZSH="$broken"; omz generate plugin partial -c partial --yes >/dev/null 2>&1 ); then
+  fail "succeeded with a missing README template"
+else
+  pass "fails when a later template is missing"
+fi
+if [[ -e "$ZSH_CUSTOM/plugins/partial" ]]; then
+  fail "partial plugin directory was left behind: $(ls "$ZSH_CUSTOM/plugins/partial")"
+else
+  pass "cleans up the partial plugin directory"
+fi
+command rm -rf "$broken"
+
 ## The prompt helper
 
 _omz::generate::plugin::ask "Q" "dflt" 2>/dev/null < <(print '')