Add a CLAUDE.md (#17517)

I'm a bit annoyed to make it tool-specific, and that it's needed in the
first place, but 🤷‍♀️ I think it'll make a difference so here we go.

This has a few items inspired by
https://github.com/astral-sh/ruff/blob/main/CLAUDE.md

CLAUDE.md

@@ -0,0 +1,11 @@
+- Read CONTRIBUTING.md for guidelines on how to run tools
+- ALWAYS attempt to add a test case for changed behavior
+- PREFER integration tests, e.g., at `it/...` over unit tests
+- When making changes for Windows from Unix, use `cargo xwin clippy` to check compilation
+- NEVER perform builds with the release profile, unless asked or reproducing performance issues
+- PREFER running specific tests over running the entire test suite
+- AVOID using `panic!`, `unreachable!`, `.unwrap()`, unsafe code, and clippy rule ignores
+- PREFER patterns like `if let` to handle fallibility
+- ALWAYS write `SAFETY` comments following our usual style when writing `unsafe` code
+- PREFER `#[expect()]` over `[allow()]` if clippy must be disabled
+- PREFER let chains (`if let` combined with `&&`) over nested `if let` statements