[FEATURE]: Plugin hook to update active model and have subagents inherit it #18666

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

Right now when a plugin changes the model via the chat.message hook, it only affects that one turn. The model picker still shows the old model, and the next turn goes back to it.

I want plugins to be able to actually change the active model so that:

• The model picker updates to show the new model
• Future turns use the new model
• Subagents inherit it properly

This would fix #17870 (subagents ignoring parent's model) and enable things like dynamic model routing based on task type.

What works today: chat.message can mutate output.message.model and it flows through to the assistant and subagents for that turn.

What's missing: The active model state and UI don't update. Future turns revert.

Proposed: When a plugin changes the model in chat.message, also update OpenCode's active model state so the UI reflects it and future turns use it.

I have tests covering the model inheritance chain at packages/opencode/test/session/chat-message-model-routing.test.ts and JSDoc in the plugin index documenting this capability.