Add marketplace.json so plugin is installable as a marketplace

[francoistatypoaty-afk]

Summary

Adds a single-plugin marketplace.json alongside the existing plugin.json so users can register this repo directly in their Claude Code settings.json as a marketplace — no separate marketplace repo required.

Why

The Claude Code plugin loader installs plugins via marketplaces. A repo that ships only plugin.json cannot be added through extraKnownMarketplaces because the loader expects a marketplace.json at .claude-plugin/marketplace.json. Without it, users have to either create a separate marketplace repo that points at this one, or clone and load via claude --plugin-dir.

Shipping a self-referential marketplace makes installation a two-line settings.json change:

"extraKnownMarketplaces": {
  "andrew-shwetzer": {
    "source": { "source": "git", "url": "https://github.com/andrew-shwetzer/career-ops-plugin.git" }
  }
},
"enabledPlugins": {
  "career-ops@andrew-shwetzer": true
}

Test plan

• Manifest validates against the marketplace schema
• plugin.json and marketplace.json agree on name, description, repository, license
• Maintainer verifies install path in a fresh session

User has explicitly authorized this PR.