core: add tool-only output mode to LLM for handoff extraction

packages/opencode/src/session/llm.ts

@@ -39,6 +39,7 @@ export namespace LLM {
     small?: boolean
     tools: Record<string, Tool>
     retries?: number
+    output?: "tool"
   }
 
   export type StreamOutput = StreamTextResult<ToolSet, unknown>
@@ -215,6 +216,7 @@ export namespace LLM {
       tools,
       maxOutputTokens,
       abortSignal: input.abort,
+      toolChoice: input.output === "tool" ? "required" : undefined,
       headers: {
         ...(input.model.providerID.startsWith("opencode")
           ? {