Adds agent md draft

[rlmark]

Adds an AGENTS.md file to be used by AI Agents. The guide emphasizes how agents should use the UCM MCP server, as some coding agents struggle with aspects of Unison.

As I understand it, this new kind of readme file describes how AI agents should behave when working in a repository, including tool expectations, unique workflows, and project-specific constraints. Also, if agents repeatedly make the same class of mistakes, we should consider adding it to this file. Not all coding tools support reading Agents.md files, but a growing subset do.

It does not cover language concepts or syntax, as that is already covered in existing files.

Considerations:

• Long files like the language guide or instructions file consume a lot of tokens. That might be necessary for some kinds of tasks, but I think these Agent files are meant to be relatively short.
• Draft PR because I'm hoping folks who use ai agents can help verify if this improves things.