A `start` or `continue` naming a `model` now reads this agent's key in the
`agent-inference` bucket and, when `subagent_models` is set, checks the
model against that list instead of HIVE_AVAILABLE_MODELS (agent.nix
`availableModels`). An unset field, an absent key or an absent bucket
keeps today's check. A read that fails or takes over 2 s falls back to
HIVE_AVAILABLE_MODELS with a warning, so a subagent start never waits on
the queue. A call without a `model` is not checked and does not read.
The read uses the agent's own queue credential through swarm_term's
connect, and the one direct-get subject the agent is granted in that
bucket. The refusal names the list and that it came from the swarm.
Refs #5034 (S5)
