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/README-generic.md-template

@@ -0,0 +1,11 @@
+# %name% plugin
+
+%description%
+
+To use it, add `%name%` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... %name%)
+```
+
+This plugin does not add any aliases.