agent_simulation: per-run concurrency + SimulationModality on dispatch

- SimulationRun.Create.Request.concurrency: max simulate jobs running in
  parallel for a run (clamped server-side to the project quota; 0 = default),
  echoed back on SimulationRun.
- SimulationModality (TEXT/AUDIO) + SimulationDispatch.modality: tells the
  agent under test how the simulated user interacts, so e.g. text simulations
  can skip STT/TTS. Simulations are text-only today; AUDIO is plumbing for
  upcoming work.

Author: theomonnom