--plugin-dir does not look for plugin.json in all the expected locations

[tmeschter]

Describe the bug

If a plugin has its plugin.json file at <plugin_root>/.claude-plugin/plugin.json and copilot is started with "copilot --plugin-dir <plugin_root>", then copilot will not load any part of the plugin nor give any diagnostic message.

Affected version

1.0.5-0

Steps to reproduce the behavior

1. Clone https://github.com/microsoft/github-copilot-for-azure.
2. Start copilot with `copilot --plugin-dir "<repo_root>/plugin".

Expected behavior

The plugin is loaded adding about 20 skills and a couple of MCP servers (including the Azure MCP).

Instead nothing happens.

Additional context

No response