feat(cli): reverse-DNS identifier, optional plugin URL, fix docs link

[vydimitrov]

Q	A
Branch?	main
Bug fix?	no
New feature?	yes
BC breaks?	no
Fixed tickets	#...

• Identifier is now reverse-DNS (e.g. com.yourcompany.plugin) instead of
  kebab-case. New src/core/helpers/plugin-identifier.ts validates input,
  suggests a com.example. default, and re-prompts/exits 2 on a bad
  value. Used by both the interactive wizard and the headless path.
• Rename the URL field/flag to "Plugin URL" / --plugin-url and make it
  optional, defaulting to http://localhost:5173/ (the dev server PORT).
  The {{vendor_url}} token is retained and now carries the plugin URL.
• Fix the post-install docs link to https://crystallize.com/docs/developer/plugins.
• Update bash completion (--vendor-url -> --plugin-url).