Remove remains of Minikanban's own Agent implementation

MR created to remove remaining references to MinikanbanAgents.

The only change of note is the use of MinikanbanAgentInterface::JOB_NOT_SOLVABLE in AgentRunner. The check before its use had already been replaced with AiAgentInterface::JOB_NOT_SOLVABLE. This has the same value (0) so there will be no change in behaviour and should have been replaced already.

Thank you for the cleanup. Merged!