Display the already existing system/user/project prompts in the settings (and allow modification)

### Feature summary

_No response_

### What problem are you trying to solve?

Currently, it is possible to edit the user level and project level instructions through the app. But when switching over from the CLI, I already have existing AGENTS.md files or ~/.copilot/copilot-instructions.md etc. So it's unclear what happens if I additionally add instructions through the app's UI. WIll the models then have those instructions from the already existing ones I had before downloading the app, clogging up the context window?

These should be discoverable and editable within the app, making it very clear what is in a particular session's context for a given repo.

Similar feedback for custom agents.

### Proposed solution

_No response_

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Start by tracing the settings UI and how it loads user-level and project-level instructions. Inspect the existing AGENTS.md and ~/.copilot/copilot-instructions.md sources, along with custom-agent handling. Done means existing instructions and agents are discoverable, editable, and clearly represented in a session's context.

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