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

[jbmml]

Issue for this PR

Fixes#20810

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The task tool spawns subtasks via SessionPrompt.prompt() but doesn't pass the parent session's thinking level (variant). The variant is already on the assistant message (msg.info.variant) and PromptInput already accepts it — this just wires them together.

The internal subtask path in prompt.ts (line 572) already does this correctly (variant: lastUser.variant). This PR makes the external task tool path consistent.

How did you verify your code works?

• Traced the variant field through the full chain: MessageV2.Assistant (line 445) → msg.info.variant (task.ts line 105) → SessionPrompt.prompt() → PromptInput.variant (prompt.ts line 1753)
• Confirmed PromptInput already accepts and uses variant (prompt.ts line 968)
• Task tool tests pass (1/1)
• Full turbo typecheck passes (13/13 packages)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate PRs found:

1. fix(opencode): Subtasks and task calls don’t inherit selected thinking level #20742 - fix(opencode): Subtasks and task calls don't inherit selected thinking level

   • fix(opencode): Subtasks and task calls don’t inherit selected thinking level #20742
   • Why related: Directly addresses the same issue - ensuring subtasks inherit the parent's thinking level variant

2. feat(task): pass variant to subagent so it inherits parent's thinking level #12567 - feat(task): pass variant to subagent so it inherits parent's thinking level

   • feat(task): pass variant to subagent so it inherits parent's thinking level #12567
   • Why related: Earlier PR that already attempted to pass variant to subagents for thinking level inheritance

You should check #20742 first since it appears to be a recent fix for the same issue that PR #20811 is addressing.

[jbmml]

Closing in favor of #20742 which is a more comprehensive fix covering the same issue across ACP, message schema, prompt, task, and tests. Our PR only addressed the task.ts path.