Add shared CLAUDE.md file

[emmaaroche]

Description

This PR adds a CLAUDE.md file to guide how Claude Code interacts with the testsuite.

The file was generated as an initial starting point by running the /init command in Claude Code from the repository root.

I’m opening this as a draft PR so we can review it as a team and discuss what to improve, remove, or add.

Closes #797

[zkraus]

You probably seen it already, but just in case, linking a anthropic's best practices https://www.anthropic.com/engineering/claude-code-best-practices

generic thoughs
Just skimmed over the initial claude.md, and my first though, it seems a bit long. If I understand it correctly, it is supplied to the session, so it's length counts to overall token cost, Just an thought, maybe think of shortening it a bit. Like making points brief, or maybe thinking of unnecessary parts. On the other hand, think of emphasizing on MUST HAVEs

[emmaaroche]

@zkraus agreed - the CLAUDE.md has been shortened (376 lines down to 120) and updated to align more closely with best practices, focusing more on the key information. I asked Claude to condense the initial file and then made a few small manual edits to it. It might still be worth making it even more concise, wdyt? 🤔

Would be great to get feedback from team to confirm if all relevant/necessary details are covered and to flag anything that might be unnecessary to keep.

Note: I’ll keep the commits separate for now so others can review the earlier version, and squash before merging when the time comes.

[averevki]

Looks good 👍

[emmaaroche]

Moved from draft to ready for review 👍

[trepel]

I am not experienced with what this documentation for AI should look like but it LGTM.