feat(plugins): add runx short name

[auscaster]

Summary

Description: Add the short name runx for the maintained Runx asdf plugin. The plugin lists only published CLI releases with complete platform archives, verifies the downloaded checksum, and installs the CLI with its companion worker when present. Its homepage is https://runx.ai.

• Tool repo URL: https://github.com/runxhq/runx
• Plugin repo URL: https://github.com/runxhq/asdf-runx

Checklist

• Format with scripts/format.bash (no unrelated changes).
• Test locally with scripts/test_plugin.bash --file plugins/runx (OK plugins/runx).
• Plugin install check is green. I also installed the current published archive with asdf on macOS and verified runx --version and the adjacent worker.