hive-subagent-mcp: check subagent models against the swarm's subagent_models

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)

hive-subagent-mcp/src/lib.rs

@@ -21,4 +21,5 @@ pub mod mcp_config;
 pub mod paths;
 pub mod role;
 pub mod session;
+pub mod swarm_models;
 pub mod swarm_term;