fix(agents): exclude subagents from UI model selection override

Subagents (explore, librarian, oracle, etc.) now use their own fallback
chain instead of inheriting the UI-selected model. This fixes the issue
where explore agent was incorrectly using Opus instead of Haiku.

- Add AgentMode type and static mode property to AgentFactory
- Each agent declares its own mode via factory.mode = MODE pattern
- createBuiltinAgents() checks source.mode before passing uiSelectedModel

src/agents/utils.ts

@@ -225,8 +225,10 @@ export async function createBuiltinAgents(
     const override = findCaseInsensitive(agentOverrides, agentName)
     const requirement = AGENT_MODEL_REQUIREMENTS[agentName]
     
+    const isPrimaryAgent = isFactory(source) && source.mode === "primary"
+    
     const resolution = resolveModelWithFallback({
-      uiSelectedModel,
+      uiSelectedModel: isPrimaryAgent ? uiSelectedModel : undefined,
       userModel: override?.model,
       fallbackChain: requirement?.fallbackChain,
       availableModels,