.github/copilot-instructions.md redirects to AGENTS.md

## User story

As a **GitHub Copilot user (Chat, PR review, or the Copilot coding agent)**, I want **`.github/copilot-instructions.md` to load automatically** so that **Copilot-generated code follows the same conventions as human- and agent-written code**.

## Why

GitHub Copilot reads `.github/copilot-instructions.md` for project-level guidance. Pointing it at AGENTS.md keeps everything in one place.

## Acceptance criteria

- [ ] `.github/copilot-instructions.md` exists.
- [ ] Its body is a one-line redirect: `See [AGENTS.md](../AGENTS.md) for repo conventions.`
- [ ] Any Copilot-specific guidance (e.g., referencing `.github/copilot/*.instructions.md` files once they exist) is appended after the redirect.

## Out of scope

- Creating the task-specific instruction/prompt files under `.github/copilot/` (separate issue).

## Notes

- Target release: **none — chore**
- Depends on `AGENTS.md` landing first.
- Source: modernization assessment §6.1.