Copilot PRs shouldn't recommend setting up copilot-instructions.md when AGENTS.md exists

[pamelafox]

Describe the bug

Copilot just made a PR for me here:
Azure-Samples/azure-search-openai-demo#2737
It includes this note:
"✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo."

However, I already have AGENTS.md set up in the repo, which y'all support now, so I think the note is unnecessary.

[nhu-do]

Great suggestion Pamela! Adding this to our internal board ✨