[Feature Request] `.copilotignore` support

**Note:** I'm not 100% sure this is the right repo for this request — please feel free to redirect if needed.

---

### Summary

It would be useful to have a way to tell the agent which files and folders it should ignore — both as a knowledge source and as targets for modification. Something similar to how `.gitignore` works, maybe called `.copilotignore`.

### Motivation

In a typical project, there are often files that exist for process/tooling reasons, archived versions, duplicates, or backups that aren't relevant to the active codebase. Right now, the agent picks these up as context, which can lead to:

- Confusion from stale or duplicate content influencing suggestions
- Unintended edits to files that should be left alone
- Noise in context that dilutes the quality of responses

### Proposed Solution

A `.copilotignore` file (or similar) at the project root that follows the same syntax as `.gitignore`. Any file or folder listed would be:

1. Excluded from being indexed or used as context by the agent
2. Protected from being modified by the agent

### Why This Matters

This would give developers more control over what the agent "sees" and touches, making it more predictable and easier to use in messy real-world projects.

---

Happy to discuss or provide more context if helpful. Thanks!