CLAUDE.md: Just copy the text from AGENTS.md in here for now.

Someone mentioned that Claude sees this file's previous text and adds "look at
AGENTS.md" to it's TODO list instead of treating it as the thing it should do
first.

If everything decides to prefer AGENTS.md at some point, we'll delete this
file.

A symlink would apparently also work, but it would be the only symlink in the
SDL git repo, so it seemed less disruptive to just copy the file over for now.

(cherry picked from commit 847fc72b1b)

CLAUDE.md

@@ -1,2 +1,22 @@
-Hi, Claude! Please read our actual instructions in the AGENTS.md file. Thanks!
+AI must not be used to generate code for contributions to this project.
+
+"AI" in this case means a Large Language Model ("LLM"), such as ChatGPT,
+Claude, Copilot, Grok, etc.
+
+AI-generated code is based upon sources of unknown origins and may not be
+compatible with the Zlib license, or may introduce conflicting license terms
+if they include code from other projects.
+
+AI can be used to identify issues with contributions to this project, but the
+solutions to those issues should be authored by humans.
+
+We have found that AI will frequently hallucinate issues that are not actually
+problems in practice, report incorrect information, and describe problems that
+are actually not issues at all. If AI identifies a problem with this codebase,
+please make sure you understand what it is saying and have independently
+confirmed that the issue exists before submitting a bug report or pull request.
+
+Any pull request to this project will ask you to confirm that you are the
+author and that you are contributing your changes under the Zlib license.
+