Adds plugins/ember/ with plugin.json and README.md so Ember
appears as an installable plugin in the awesome-copilot
marketplace. The agent and skill files already exist at the
repo root from PR #1324.
Ran npm run plugin:validate (passes) and npm start to
regenerate README and marketplace.json.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
