Function init_logging 

pub(crate) fn init_logging(log_dir: &Path) -> WorkerGuard

Initialize tracing with dual output: rolling daily log file + human-readable stderr. log_dir is resolved by the caller: <portable-root>/data/logs/ in portable mode, ~/.agentmux/logs/ in installed mode. Returns a guard that must be held for the lifetime of the process to ensure log flushing.