docs: add shared agent instructions

CLAUDE.md

@@ -0,0 +1,11 @@
+@AGENTS.md
+
+## Claude Code
+
+Follow `AGENTS.md` as the shared repository instructions.
+
+Do not duplicate contribution, style, PR, dependency, or security policy here;
+use `CONTRIBUTING.md` as the canonical source for those rules.
+
+Keep Claude-specific project notes in this file only when they do not apply to
+other coding agents.