Ignore AI agents related files

Developers can use AI agents when developing Ruby, but we don't want to
commit the AI agents related files to the repository by accident.

Author: st0012

.gitignore

@@ -278,3 +278,8 @@ lcov*.info
 /bundled_gems.json
 /default_gems.json
 /gems/default_gems
+
+# AI agents
+/.claude
+/AGENTS.md
+/CLAUDE.md