Plugins installed via a Marketplace aren't available via ACP

### Describe the bug

I've got a number of plugins that are installed via a local clone of a marketplace repository. In the CLI, I can see all of the plugins and use the agents and skills contained within. When accessing Copilot via ACP, the models are unaware of any plugins, so the agents and skills are no longer available.

### Affected version

1.0.21

### Steps to reproduce the behavior

1. Have Copilot CLI setup with plugins installed from a marketplace. Confirm that you can use the agents and skills in the CLI.
2. Via ACP, try and invoke an agent or skill. It fails; the model reports that it's not aware of that agent or skill.
3. Via ACP, request a list of "custom agents". It will return agents from the .github/agents/ directory, but not those from the plugins.
4. Via ACP, request a list of plugins. It will tell you that there are no plugins.

### Expected behavior

For 2, it should use the agent or skill.
For 3, it should return a list of custom agents from the plugins.
For 4, it should return a list of plugins.

### Additional context

_No response_

No implementation files or tests are named. Start by tracing how the CLI discovers plugins from a local marketplace and how ACP exposes agents, skills, and plugin listings; compare those paths. Done means ACP can use marketplace agents and skills and lists the plugins, matching the reported CLI behavior.

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