v0.10.0 — sub-agent model overrides + graceful no-model UX

Features

• Per-invocation sub-agent model overrides (port of upstream mpfaffenberger#377): new invoke_agent_with_model + list_available_models tools let routing agents (e.g. the Orchestrator) run a sub-agent on a specific model for a single call — route cheap/mechanical steps to a fast model, reserve a strong model for hard fixes. Built on BaseAgent.temporary_model_name_override (precedence: runtime → pinned → global); JSONAgent honors it too. The chosen model is surfaced in AgentInvokeOutput.model_name and the invocation UI.
• Graceful no-model handling: get_global_model_name() now returns None instead of a phantom gpt-5; the statusline shows [None]; a one-shot warning points at /add_model.
• Onboarding nag: after the wizard, if no model is configured (and OAuth wasn't chosen), prompts to run /add_model.
• models.json ships empty: first run requires explicit model setup via /add_model or OAuth. Existing users' extra_models.json / OAuth models still merge in automatically.