Allow plugins to include instruction files

### Describe the feature or problem you'd like to solve

In order to share instruction files, you would have to send it to your team (eg through git) then they would need to add it to their ~/.copilot/ folder and perform that task every time you made an update to the instruction.

### Proposed solution

Allow plugins to include instruction files that are automatically loaded into agent context when the plugin is installed.
Organisations can share internal best practises through instruction files and update them through plugins.

### Example prompts or workflows

Language specific plugins (eg C#) could include an instruction file that explains best practices and code formatting.
The current C# plugin from awesome-copilot uses skills instead, which are not always invoked by the agent.

Organisations can write instruction files that detail company specific best practices. The instruction files can be kept up to date and managed through an internal marketplace repo.

### Additional context

_No response_

issue 中沒有指定檔案、測試或進入點。首先追蹤 plugin 的安裝方式，以及目前如何探索或載入指示檔案，然後確定涉及的 plugin 中繼資料與載入邊界。完成的標準是：已安裝的 plugin 可以提供指示檔案，這些檔案會自動包含在 agent context 中，並且仍可透過 plugin 更新。

由索引模型根據 Issue 內容生成。