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/explore.ts

@@ -1,7 +1,9 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
-import type { AgentPromptMetadata } from "./types"
+import type { AgentMode, AgentPromptMetadata } from "./types"
 import { createAgentToolRestrictions } from "../shared/permission-compat"
 
+const MODE: AgentMode = "subagent"
+
 export const EXPLORE_PROMPT_METADATA: AgentPromptMetadata = {
   category: "exploration",
   cost: "FREE",
@@ -34,7 +36,7 @@ export function createExploreAgent(model: string): AgentConfig {
   return {
     description:
       'Contextual grep for codebases. Answers "Where is X?", "Which file has Y?", "Find the code that does Z". Fire multiple in parallel for broad searches. Specify thoroughness: "quick" for basic, "medium" for moderate, "very thorough" for comprehensive analysis. (Explore - OhMyOpenCode)',
-    mode: "subagent" as const,
+    mode: MODE,
     model,
     temperature: 0.1,
     ...restrictions,
@@ -119,4 +121,4 @@ Use the right tool for the job:
 Flood with parallel calls. Cross-validate findings across multiple tools.`,
   }
 }
-
+createExploreAgent.mode = MODE