Nothing capped how many subagents an agent runs at once; the operator
wants to set that per agent from swarm-ui (#5234, shape (b)).
AgentInference gains `max_subagents: Option<u32>`. An absent field
decodes as None, so every stored value still parses and means "no cap",
which is today's behaviour.
swarm-controller accepts 1..=32 on PUT. 0 would refuse every start; 32
is far more concurrent claude sessions than one agent container has
memory for, so anything above it is a typo, not a cap.
The subagent daemon already read the agent's agent-inference key for
subagent_models; it now reads it on every start and continue (not only
those naming a model) and refuses a new run once the cap's worth of
names is tracked, counted under the same lock as the name reservation.
Every tracked slot counts: running, starting, a goal run between turns,
and an interrupted one still exiting. A goal re-prompt never goes
through the reservation, so it is never refused. A failed read applies
no cap, as it falls back for models.
swarmctl's `inference set` does GET, merge, PUT; its mirror type now
carries the field so a set from the CLI keeps a cap set in swarm-ui
instead of dropping it, and `get` prints it.
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