Document Anthropic advisor rosters and pinned inference regions

[kaxil]

Summary

Anthropic SDK 0.121.0 adds two agent-level settings: an advisor entry for a coordinator's
multiagent roster, and inference_geo on the model config for pinning which region serves
inference. Both reach the API through AnthropicHook.create_agent's existing keyword
passthrough, so they need no provider support -- only documentation, since nothing in the
guide says they are available.

Pinning the inference region, which is what a data-residency requirement usually comes down
to:

hook.create_agent(
    name="eu-only-analyst",
    model={"id": "claude-opus-4-8", "inference_geo": "eu"},
)

Adding an advisor the coordinator may consult mid-turn:

hook.create_agent(
    name="coordinator",
    model="claude-opus-4-8",
    multiagent={
        "type": "coordinator",
        "agents": [worker_agent_id, {"type": "advisor", "model": "claude-opus-4-8"}],
    },
)

Why the shapes are verified rather than read off the types

Both payloads were exercised against the live API. That mattered: the first attempt omitted
multiagent.type: "coordinator" and was rejected with multiagent.type: Field required, so
documenting from the SDK type definitions alone would have shipped a payload that 400s.

The guide also records the constraints the API enforces (1 to 20 distinct roster entries, at
most one advisor, referenced agents may not themselves set multiagent) and that model is
whole-object replacement on update, so omitting inference_geo clears it rather than
preserving it.

One code change

create_agent's model annotation widens from str | None to accept a mapping. It already
worked at runtime, but a user type-checking their Dag would have got an error on the exact
snippet the guide tells them to write.

[Lee-W]

one nit