feat(cli): add omz generate plugin to scaffold custom plugins

Adds a rails-style generator that creates a custom plugin in
$ZSH_CUSTOM/plugins/<name> from commented templates: the plugin file,
a README in the usual shape and, for plugins that wrap a command-line
tool, a completion skeleton (or the cached-completion block when the
tool generates its own). Anything not passed as an option is asked
for interactively; non-interactive shells use defaults.

`omz generate` is a new top-level command so that other generators
(e.g. `omz generate theme`) can follow the same shape.

Templates live in templates/generators/plugin and are syntax-checked
by CI. lib/tests/generate-plugin.test.zsh covers the non-interactive
path, name validation and the prompt helper.

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

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

@@ -0,0 +1,22 @@
+#
+# Completion
+#
+# %command% generates its own zsh completion, so we cache it instead of
+# maintaining a `_%command%` file by hand.
+#
+# On the first shell after installing this plugin the cache file doesn't exist
+# yet, so compinit hasn't bound it. Do that manually here.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_%command%" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _%command%
+  _comps[%command%]=_%command%
+fi
+
+# Regenerate the cache in the background so it never blocks shell startup.
+# TMPPREFIX puts the temp file next to the destination, which keeps the move
+# atomic.
+zmodload -F zsh/files b:zf_mv
+() {
+  local TMPPREFIX="$ZSH_CACHE_DIR/completions/_%command%"
+  zf_mv -f -- =( %compgen% ) "$TMPPREFIX"
+} &|