Run Bash through a login shell

The Bash tool invokes model-authored source with bash -lc, so commands receive the user's normal login environment and command lookup. Login startup files execute before the requested source and are treated as trusted user environment initialization rather than input to the model-command classifier. They remain inside the invocation's selected confinement, timeout, process group, and output capture, so adopting login-shell behavior does not create a separate execution path.