Stock macOS and Debian ship python3 only. Nothing named `python` is on PATH there, so the bundled antislop-contrast MCP server never starts and the command the antislop-human skill prints fails the same way. Changes `command` in plugin.json, the allowed-tools Bash rule and the example in antislop-human/SKILL.md, and the usage strings in contrast-check.py. The script's own shebang was already python3.