mise: Local credential_command executes untrusted config

mise manages dev tools like node, python, cmake, and terraform. From 2026.3.15 until 2026.6.4, mise loads github.credential_command from local project config before any trust decision, then executes that value with sh -c when resolving a GitHub token. An attacker who can place a .mise.toml in a repository can execute arbitrary shell commands when the victim runs a GitHub-related mise command and no higher-priority GitHub token environment variable is set. This vulnerability is fixed in 2026.6.4.