Shell step streaming: when LogSink is present, uses cmd.spawn() with tokio::select! to interleave stdout/stderr line-by-line. Respects timeout_ms with child.kill() on timeout. Falls back to buffered mode when no LogSink. Security: block sensitive env var overrides (PATH, LD_PRELOAD, etc.) from workflow data injection. Proper error handling for pipe capture. 4 LogSink regression tests + 2 env var security regression tests.