`Reader` gets a test-only fixed source that counts its reads, and `State`
takes the agent's own list as a `fn` (`available_models` outside tests), so
`start` and `continue_` can be driven through `State::allow_model` without
a queue or a process-wide env change. The new tests pin that both calls
refuse a model outside the swarm list ahead of `reserve`, accept a model
the swarm lists but the local list lacks, and read nothing for a call
without a `model`.
Refs #5034 (S5)
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)
