chore: Add AGENTS.md, symlink copilot instructions

[elliott-with-the-longest-name-on-github]

I've been playing around with Claude and Opencode, and they utilize AGENTS.md pretty effectively. If we add a code review bot in the future, it'll also utilize it. I went ahead and synthesized the stuff from .github/copilot-instructions.md with some other stuff, moved it to AGENTS.md, and symlinked .github/copilot-instructions.md to AGENTS.md.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15145-svelte.vercel.app/

[Conduitry]

On Windows, in order for cloning this repo as actual symlink, you will need to (1) set the appropriate git config to enable core.symlinks, and then (2) set your computer to be in 'developer mode' so that you can create symlinks without elevated privileges (for whatever reason, they require elevated privileges by default). Are we concerned about this?

[elliott-with-the-longest-name-on-github]

That's... insane. Another L for Windows, I guess. Maybe we can just make the contents of the symlinked file "please read this file instead" 😆

[Conduitry]

Is copilot-instructions something that's likely to be read on people's locally checked-out copies of the code, or just in the GitHub web interface? If it's not going to be read locally, I'd actually tend to not worry about this. If it is going to be looked at locally, then ... I'd probably still also be okay with this, but might want to check with Simon who is probably the only maintainer who regularly works on Svelte on Windows and not under WSL.

[mquandalle]

Claude code doesn't support AGENTS.md anthropics/claude-code#6235, but only CLAUDE.md. It's possible to import another file using @import

[elliott-with-the-longest-name-on-github]

Given they have literally thousands of thumbs-ups and virtual universal positive sentiment on that GitHub issue, I'm inclined to say "guess they'd better support AGENTS.md then" 😆 I think the "create a CLAUDE.md that imports AGENTS.md" workaround is fine for now, though

[elliott-with-the-longest-name-on-github]

@Conduitry sorry, missed that comment -- for VSCode users who use Copilot chat, it likely will be read. Given the issue with CLAUDE.md below though I'm inclined to use natural language rather than a symlink just to make it easier

[elliott-with-the-longest-name-on-github]

Nah, nevermind. If Windows users end up having a problem with it, we can consider using a different strategy. Otherwise... symlinks are literally for pointing files at other files.

[mquandalle]

Suggested change

	AGENTS.md
	@AGENTS.md

Otherwise it isn't automatically included

[elliott-with-the-longest-name-on-github]

Nah, this is just GitHub's weird way of rendering symlinks. This is a symlink to AGENTS.md so when Claude reads the file it'll get the full contents of AGENTS.md without the intermediate step of having to read the file and @import it