Do not override the subagent model

Do not set the model parameter on Agent tool calls. Let subagents inherit the model from the parent session.

Why: User sets their preferred model via /model command. Overriding it with a different model is unexpected and unwanted.

How to apply: Never pass model to Agent invocations unless the user explicitly asks for a specific model on a subagent in this turn.

Emacs 29.1 (Org mode 9.6.6)