Documents why the supervisor suite stays sequential (claim is cross-tenant via LoadNextPendingForUpdateSkipLocked; parallel supervisors would steal each other's runs) and switches the SIGTERM subtest to errors.AsType[*exec.ExitError] to match the project convention enforced elsewhere in this file. Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>