Add AGENTS.md to guide AI coding assistants

[planetf1]

AI coding assistants (Claude Code, Cursor, Copilot, etc.) working with Mellea could produce code that doesn't follow project patterns - wrong import paths, missing type hints, tests without proper markers.

There's a convention called AGENTS.md that provides structured guidance for AI assistants, similar to how .editorconfig helps editors and CONTRIBUTING.md helps human contributors.

References:

• https://docs.anthropic.com/en/docs/claude-code/memory#agentsmd
• https://cursor.directory/

Why this would help:

• AI assistants would use @Generative instead of raw OpenAI calls
• Correct import paths (mellea.stdlib.requirements not mellea.stdlib.requirement)
• Tests get proper markers (qualitative, integration)
• Code follows project conventions (types, docstrings, ... in generative bodies)

What's needed:

• AGENTS.md in repo root for contributors working on Mellea internals
• docs/AGENTS_TEMPLATE.md that downstream projects can copy when building apps with Mellea

I have a PR I'll propose which it would be great to get feedback on.