[FEATURE]: add chat.model plugin hook for pre-call model routing

[v1truv1us]

• I have verified this feature I'm about to request hasn't been suggested before.

Add a new plugin hook named chat.model that runs before an LLM call and can replace the active { providerID, modelID }.

Today plugins can modify params, headers, prompts, and tool behavior, but they cannot choose a different model at call time. That makes it hard to build plugin-driven routing patterns that feel like a good fit for OpenCode: cost-based switching, fallback to another model, per-tool routing, and review workflows that use multiple configured models without manually changing the session model.

I think this hook should stay minimal. It only needs the current call context: sessionID, agent, current model, provider context, current message, and step. It should return an optional replacement model and does not need full conversation history.

This seems related to fallback discussions like #7602, but broader: the goal is to expose model routing to plugins, not only add native fallback behavior. Since this changes core plugin infrastructure, I think it should start as a design discussion before implementation.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Plugin can change the active model and have it stick #18667: Open feature request by the same author asking for plugins to change the active model and have it persist across turns — very similar goal of plugin-driven model routing
• [FEATURE]: Dynamic Model Routing via Plugin Hook - Update Active Model State #18644: Closed feature request (also by the same author) proposing a chat.model hook and dynamic model routing via plugin hook — nearly identical to this request

Please review those issues and consider whether this is addressing something distinct before proceeding.

[v1truv1us]

This is similar to the other issue, and from what I can see doing this would probably help fix it, but it's not the same problem