Salvages #22981 by @SimbaKingjoe. Adds 'kanban.max_in_progress' config
that caps simultaneously running tasks. When the board already has N
running, dispatcher skips spawning so slow workers (local LLMs,
resource-constrained hosts) don't pile up and time out.
Threads through dispatch_once(max_in_progress=) and gateway dispatcher
config parsing with validation (warns on invalid/below-1 values).