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_

No file, test, or entry point is named in the issue. Start by tracing how plugins are installed and how instruction files are currently discovered or loaded, then determine the plugin metadata and loading boundaries involved. Done means an installed plugin can provide instruction files that are automatically included in agent context and remain updateable through the plugin.

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