Add AGENTS.md (#18902)

## Summary

Like Ruff, we now use `AGENTS.md` and tell Claude to look at that for
any Claude users.

AGENTS.md

@@ -0,0 +1,19 @@
+- 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
+- NEVER update all dependencies in the lockfile and ALWAYS use `cargo update --precise` to make
+  lockfile changes
+- NEVER assume clippy warnings are pre-existing, it is very rare that `main` has warnings
+- ALWAYS read and copy the style of similar tests when adding new cases
+- PREFER top-level imports over local imports or fully qualified names
+- AVOID shortening variable names, e.g., use `version` instead of `ver`, and `requires_python`
+  instead of `rp`
+- PREFER [`TypeName`] references when writing Rust doc comments

CLAUDE.md

@@ -1,19 +1 @@
-- 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
-- NEVER update all dependencies in the lockfile and ALWAYS use `cargo update --precise` to make
-  lockfile changes
-- NEVER assume clippy warnings are pre-existing, it is very rare that `main` has warnings
-- ALWAYS read and copy the style of similar tests when adding new cases
-- PREFER top-level imports over local imports or fully qualified names
-- AVOID shortening variable names, e.g., use `version` instead of `ver`, and `requires_python`
-  instead of `rp`
-- PREFER [`TypeName`] references when writing Rust doc comments
+@AGENTS.md