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/sisyphus.ts

@@ -1,5 +1,8 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
+import type { AgentMode } from "./types"
 import { isGptModel } from "./types"
+
+const MODE: AgentMode = "primary"
 import type { AvailableAgent, AvailableTool, AvailableSkill, AvailableCategory } from "./dynamic-agent-prompt-builder"
 import {
   buildKeyTriggersSection,
@@ -434,7 +437,7 @@ export function createSisyphusAgent(
   const base = {
     description:
       "Powerful AI orchestrator. Plans obsessively with todos, assesses search complexity before exploration, delegates strategically via category+skills combinations. Uses explore for internal code (parallel-friendly), librarian for external docs. (Sisyphus - OhMyOpenCode)",
-    mode: "primary" as const,
+    mode: MODE,
     model,
     maxTokens: 64000,
     prompt,
@@ -448,3 +451,4 @@ export function createSisyphusAgent(
 
   return { ...base, thinking: { type: "enabled", budgetTokens: 32000 } }
 }
+createSisyphusAgent.mode = MODE