Plugin-provided extension canvases are unavailable in general chat sessions

### Feature summary

_No response_

### What problem are you trying to solve?

### Summary

A canvas extension bundled with an installed Copilot plugin cannot be opened from a general chat session. The extension is only discovered when the chat runs inside a project containing the extension under `.github/extensions`.

The plugin and its skills load successfully, but its bundled canvas does not.

### Reproduction

1. Install and enable a plugin containing a canvas extension such as `.github/extensions/report-viewer/extension.mjs`.
2. Declare the extension directory in the plugin manifest.
3. Start a general Copilot chat not attached to a project session.
4. Invoke a plugin skill that calls `open_canvas`.
5. The call fails:

```text
Canvas "project:report-viewer/report-viewer" is not registered.
No canvas "report-viewer" is registered.

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

首先跟踪插件清单如何暴露 .github/extensions/report-viewer/extension.mjs，以及通用聊天如何处理 open_canvas 调用。将该路径与能够发现 canvas 的项目会话进行比较。完成的标准是：启用插件中捆绑的 canvas 已注册，并且可以从通用聊天中打开，同时现有的项目会话行为保持不变。

由索引模型根据 Issue 内容生成。