The playbook did not use to define a prompt for statically-defined
agents.

Since prompt variables support landed in v1.1.0
(see 2a5a2d6a4d)
it makes sense to make use of it for a better out-of-the-box experience
(see https://github.com/etkecc/baibot/issues/10).

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3369