Add CLAUDE.md for Claude Code guidance

[tilladam]

Provides build commands, testing instructions, architecture overview, and platform prerequisites to help Claude Code navigate the repository.

[CLAassistant]

All committers have signed the CLA.

[NigelBreslaw]

Any thoughts about the more generic AGENTS.md? I'm not sure how standardised this area is as my hobby projects have ended up with .cursorrules and AI_INSTRUCTIONS.md.

[tilladam]

I have no opinion on the matter, more generic is better. I guess we could (lol) ask the LLMs what the consensus is?

[NigelBreslaw]

There is something else we (me?) failed to document. But you can use mise within the monorepo and it will download and install quite a few tools and the correct rust compiler, but only within the monorepo itself. It won't mess with your entire machines dev environment. So the AGENTS.md text could have something like "Install Mise. On MacOS brew install mise. Then mise install to set it up. When making changes as part of verifying they work run mise run 'ci:autofix:fix'". Which runs the same autofix task as the Slint CI, but locally."

[tilladam]

There doesn't seem to be a consensus way to do this yet, the other tools seem to also parse CLAUDE.md (and various other files). I'd say let's merge this and add mise into a more general file in docs/ that is linked from CLAUDE.md also? We can also extract more stuff into docs/building.md etc, if we want. The bigger question is, do we want this in the repo to help save tokens amongs users of Claude?

[tronical]

Olivier and I discussed this. We don't mind an AGENTS.md file in the top-level, but it could also go into another directory if suitable. It seems valuable to have something like this and the continue refining it as the tooling evolves.

This PR needs a rebase so that only the newly added file shows up in the diff.

[tronical]

(GH requires a comment when giving a review)

[NigelBreslaw]

It's the root or not at all. Part of requesting 'AGENTS' instead of CLUADE was too minimise it to a single file and not one for each flavour of the month LLM.

[tronical]

Yep, top-level (root) is fine with Olivier and me. Whatever works :)