Completes the maintainer's removal of per-call model selection from
subagent spawns (9f8aa87dbf removed it from the task tool and the
model-facing agent() docs/prompt, but the eval agent() runtime and all
four preludes still accepted and forwarded a per-call model).
Subagents now always resolve through the selected agent's frontmatter
model and settings, so an explicit model: "default" can no longer
silently route children onto the parent session model.
- agent-bridge: drops "model?" from agentArgsSchema and the request
forward; adds "+": "delete" so a legacy model argument is stripped
(same contract as the task wire schemas).
- JS/Python/Ruby/Julia preludes: remove the model parameter from
agent(); completion()'s tier selector is unchanged.
- docs (tools/eval.md, python-repl.md) updated to the removed surface.
Refs #6438
Slava Zavadsky
b8779dae63