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/main.rs

@@ -56,7 +56,8 @@ async fn main() -> Result<()> {
     let state = Arc::new(
         hive_subagent_mcp::session::State::new(todo_socket, signal_base)
             .on_runtime(runtime)
-            .publishing_to(hive_subagent_mcp::swarm_term::Publisher::spawn()),
+            .publishing_to(hive_subagent_mcp::swarm_term::Publisher::spawn())
+            .reading_models_from(hive_subagent_mcp::swarm_models::Reader::spawn()),
     );
 
     // Serve the MCP tools over streamable-http forever. No background poll