Add GitHub Action to notify PR authors of repo migration and auto-close PRs

[Copilot]

Most logic in this repo is migrating to alphaonelabs/learn. New PRs opened here need to be redirected there.

Changes

• .github/workflows/pr-migration-notice.yml — new workflow triggered on pull_request_target: [opened] that:
  • Posts a comment tagging the author, explaining the migration, linking to alphaonelabs/learn, and instructing them to adapt and resubmit there
  • Immediately closes the PR automatically

The comment makes clear the PR is already closed (no ambiguous "let us know" language):

## 🚚 This Repository Is Moving

Hi @author, thank you for your contribution!

We are in the process of migrating most of the logic from this repository to
alphaonelabs/learn.

1. Review the alphaonelabs/learn repository and familiarize yourself with its tech stack.
2. Adapt your changes to work with that codebase.
3. Open a new Pull Request in alphaonelabs/learn.

This PR has been automatically closed. Once you have opened your PR in the
new repository, feel free to reference it here.

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