Add CODEOWNERS and AGENTS.md agent guidance

[dduugg]

Summary

• Adds .github/CODEOWNERS assigning the whole repo to @rubyatscale/ruby-at-scale. Every other rubyatscale repo that has a CODEOWNERS file uses this same line.
• Adds AGENTS.md, guidance for AI coding agents working in this repo. It covers:
  • how action.yml, action.sh, and support/ fit together;
  • the bash 3.2 constraint on action.sh (macOS runners);
  • the shellcheck, actionlint, and zizmor commands, plus how to run action.sh locally against the test fixtures;
  • the version and checksum pinning conventions (Dependabot, Renovate's custom regex managers, and the manual checksum refresh).
• Adds CLAUDE.md, which only imports @AGENTS.md, so Claude Code and other agents read the same file.

Testing

I ran every command in AGENTS.md locally. shellcheck, actionlint (with the $/ ignore), and zizmor --offline . all report nothing. /bin/bash ./action.sh (bash 3.2) passes on formatted.rb, and on unformatted.rb it exits 5 with outcome=failure.