🤖 feat: split Exec sub-agent AI defaults

docs/agents/index.mdx

@@ -185,6 +185,17 @@ task({
 
 Only agents with `subagent.runnable: true` can be used this way.
 
+### Run-context AI defaults
+
+The same agent identity can use different default model and thinking settings depending on how it runs:
+
+- **UI defaults** (`agentAiDefaults`) apply when you select the agent directly in the UI, such as choosing Exec in the chat input.
+- **Subagent defaults** (`subagentAiDefaults`) apply when that agent is spawned through the `task` tool.
+
+Subagent defaults inherit from UI defaults per field. If the subagent model is unset, Mux uses the matching UI agent model; if subagent thinking is unset, Mux uses the matching UI agent thinking level. You can override one subagent field and keep the other inherited.
+
+Mux resolves the subagent model and thinking level when the `task` call creates the child workspace. Those resolved values are stored with that child workspace, so changing defaults later affects future subagent tasks only.
+
 ## Examples
 
 ### Security Audit Agent