Drop .github/copilot-instructions.md in favour of root AGENTS.md

Agent runtimes read AGENTS.md natively, so a separate Copilot-only pointer file is redundant duplication. Repository-Defaults still lists it; tracked in PSModule/docs#102.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug