Dynamically supervises individual agent executions.

A DynamicSupervisor (strategy: :one_for_one) started once under this application's supervisor. Every call to start_run/1 adds one BeamAgent.Runner child with restart: :temporary — a run that finishes, fails, or crashes is simply gone; it is never automatically restarted, since a completed or crashed run is a terminal result to hand back to the caller, not a transient failure to retry. Because children are dynamic and independent, any number of runs can be active at once.