fix(opencode): forward parent attachments to subagents

[21pounder]

Issue for this PR

Closes #25553

Type of change

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

What does this PR do?

This fixes the attachment handoff for @mention subagents in the task path.

In the failing #25553 flow, the parent agent could spawn the subagent, but TaskTool only rebuilt child prompt parts from params.prompt, so the image attached to the parent user message never reached the child session.

This change forwards file parts from the triggering parent user message into the child prompt, but only when that parent message explicitly targets the same subagent. That keeps the fix scoped to the actual handoff boundary and lets multimodal subagents receive the attached image once they are spawned.

This does not try to solve cases where the parent LLM does not choose to call task at all. That routing decision is upstream model behavior, not something this patch can make deterministic.

sequenceDiagram
    participant U as User
    participant P as Parent session
    participant B as build model
    participant T as TaskTool
    participant V as Child subagent

    U->>P: text + @vision + image
    P->>B: parent message
    B->>T: call task(subagent, prompt)
    T->>V: child prompt from params.prompt only
    Note over V: image missing

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sequenceDiagram
    participant U as User
    participant P as Parent session
    participant B as build model
    participant T as TaskTool
    participant V as Child subagent

    U->>P: text + @vision + image
    P->>B: parent message
    B->>T: call task(subagent, prompt)
    T->>T: collect file parts from the triggering parent user message
    T->>V: child prompt + forwarded image
    V-->>P: image analysis result

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How did you verify your code works?

• Sent one POST /session/:id/message request with text + agent + file parts
• Confirmed via GET /session/:id/message that the session stored the @vision mention and the image attachment
• Confirmed in the server log that a child session was created with agent=vision
• Confirmed in the server log that the child session streamed with agent=vision and mode=subagent
• Ran bun typecheck from packages/opencode
• Ran bun test --timeout 15000 test/tool/task.test.ts from packages/opencode

Screenshots / recordings

POST request with text + agent + file parts:

Stored session message with @vision and image attachment:

Child session creation for the subagent:

Child subagent stream:

Regression test passing:

Checklist

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

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