Function apply_gh_guard_in 

pub fn apply_gh_guard_in(
    env_vars: &mut HashMap<String, String>,
    config_home: &Path,
)

Point GH_CONFIG_DIR at this agent’s login-free directory, under config_home. The agent is read from the env itself (AGENTMUX_AGENT_SLUG, then AGENTMUX_AGENT_ID).

Never fails the spawn: if the directory can’t be prepared the variable is still set, and a missing directory is just as logged-out as an empty one.