feat: add support for the fabrica coding agent

[Endi1]

This PR adds support for my personal minimalistic coding agent harness: https://github.com/Endi1/fabrica (ACP support is through an ACP server built into the agent itself)

Since I mostly use any coding agents from emacs through agent-shell, it would be nice if it'd have official support for fabrica in case the number of users increases to more than one, and if not, I'd be happy to maintain a fork for it anyway :)

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I'm making visual changes, so I'm including screenshots so you can view and discuss.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[xenodium]

Thanks for the PR. Could you share a little more about the state of the agent? Do you consider it ready for wider adoption? Known limitations, specially with agent-shell? I probably won't have time to try it out myself (due to #500), but can you post some screenshots so I can at least see the integration with welcome screen and graphical header?