fix(opencode): inherit thinking level (variant) in task tool subtasks

The task tool was not passing the parent session's thinking level
to spawned subtasks. The variant field was available on the assistant
message but was not forwarded to SessionPrompt.prompt(), causing all
subtasks to use the default thinking level.

Fixes #20810

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: Joey Barkley

packages/opencode/src/tool/task.ts

@@ -135,6 +135,7 @@ export const TaskTool = Tool.define("task", async (ctx) => {
           providerID: model.providerID,
         },
         agent: agent.name,
+        variant: msg.info.variant,
         tools: {
           ...(hasTodoWritePermission ? {} : { todowrite: false }),
           ...(hasTaskPermission ? {} : { task: false }),