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)

docs/swarm/inference.md

@@ -16,8 +16,11 @@ swarmctl agent inference clear iris
 - `set` changes only the flags you pass; the other fields keep what's
   stored. A hive must declare the agent.
 - `--effort` is one of `low`, `medium`, `high`, `xhigh`, `max`.
-- `--subagent-models` takes a comma-separated list. It's stored and shown,
-  but agents don't apply it yet.
+- `--subagent-models` takes a comma-separated list. It replaces the
+  agent's `availableModels` as the models its subagent `start` and
+  `continue` calls may name, read on each call
+  ([`subagent.md`](../tools/subagent.md#which-models-a-subagent-may-run-on)).
+  The web UI's model picker doesn't use it.
 - `get` shows each field, naming the unset ones.
 - `clear` removes every field; the agent goes back to its own values.
 
@@ -65,4 +68,8 @@ The harness (`hive-agent`'s `swarm_inference`) applies a set field to the
 in-memory model or effort without writing `hyperhive-model` or
 `hyperhive-effort`, which keep the agent's own value.
 
+The subagent daemon (`hive-subagent-mcp`'s `swarm_models`) reads
+`subagent_models` with the same direct get, under the same credential, on
+each `start` or `continue` that names a model.
+
 </details>