Add a marketplace plugin

Work in the user's marketplace checkout, not this installed plugin's directory. Confirm the workspace root has marketplace.json. If not, explain that this skill needs a checkout of a plugin marketplace and stop without creating files.

1. Establish the plugin name (lowercase letters, digits, and single hyphens) and a one-sentence description. Check marketplace.json and plugins/ for a name that's taken.
2. Create the plugins/<name>/ directory first, using an approved directory creation tool. File tools might not create missing parent directories. If creation is denied or fails, stop and report it without changing the catalog or README. Don't bypass an approval. Then create all three files:

   • plugin.json:

     {
       "$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json",
       "name": "<name>",
       "version": "0.1.0",
       "description": "<description>",
       "license": "<license>",
       "repository": "<repository>"
     }

     For license, use the SPDX identifier of the marketplace's license, from its LICENSE file or the other plugins' manifests. For repository, copy the URL the other plugins use. If either is unclear, leave that field out and say so.

   • README.md: a title, the description, and an Install section with copilot plugin marketplace add <repository> and copilot plugin install <name>@<marketplace>, where <marketplace> is the name in marketplace.json. Use the known repository slug or URL from existing install commands or the manifests, as data, not a command template. Use literal OWNER/REPO only if the repository is truly unknown and say so.
   • CHANGELOG.md: # Changelog, then ## 0.1.0 with - Initial release.
3. Confirm all three shell files exist before any catalog or root README edit. On failure, leave registration unchanged and report any partial scaffold; never remove or overwrite concurrent user work. Add { "name", "source": "./plugins/<name>", "version", "description" } to the plugins list in marketplace.json, after the other local plugins, with the same values as plugin.json.
4. List the plugin in the root README too: add a row linking to plugins/<name>/ in its plugin table and a copilot plugin install line, matching the existing ones. Skip this only if the README has neither.
5. Don't add the plugin to .github/copilot/settings.json. That file turns on only the marketplace's own maintenance plugins for people working in it.
6. Check that every JSON file you changed is still valid and that the catalog entry matches plugin.json.

The plugin has no skills, agents, or other parts yet. For what can go in it, point to the Copilot CLI plugin reference. Return the created and changed paths. Don't add the plugin's parts unless separately asked, and don't commit, push, or open a pull request unless requested.