## Why
The `agent_type` argument is only useful when agent roles are configured. Avoid
advertising an inapplicable override in the model-visible `spawn_agent` schema.
## What changed
- Include `agent_type` in the V1 and V2 spawn tools only when configured roles
are available.
- Clarify that `agent_type` is an explicit override and document its interaction
with context forking.
- Remove the unused `agents.support_agent_type` configuration field.
## Testing
- Add coverage for hiding `agent_type` without roles and exposing it when a role
is configured.
GitOrigin-RevId: 32e0f123b15f317e9cdb71774029375bc073d9ad