CLAUDE.md: added.

Apparently Claude Code won't read AGENTS.md by default at this moment, but
since that's the direction we're moving, let's just add the file Claude
currently looks for and tell it that the actual instructions are in AGENTS.md.

I thought these things were supposed to be less complicated that interacting
with humans, lol.

(cherry picked from commit 1124e44e4c66668a719147a2032c5e77015583c8)

CLAUDE.md

@@ -0,0 +1,2 @@
+Hi, Claude! Please read our actual instructions in the AGENTS.md file. Thanks!
+