fix: preserve instance context for async prompts

[qz1543706741]

Issue for this PR

Closes #26526

Type of change

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

What does this PR do?

promptAsync starts prompt work in a forked background effect after returning 204.

This change captures the request's InstanceRef and WorkspaceRef before forking, then provides them to the background prompt effect. That keeps async prompt execution on the same instance/workspace context as the HTTP request, so project-specific config such as custom agents remains available.

How did you verify your code works?

• Confirmed the patch is limited to packages/opencode/src/server/routes/instance/httpapi/handlers/session.ts.
• Ran git diff --check successfully.
• Ran bun typecheck from packages/opencode; it starts successfully but currently fails on upstream dev with an unrelated existing error in src/bus/global.ts (GlobalBusEmitter.emit signature mismatch). This change does not touch that file.

Screenshots / recordings

Not applicable. This is a server-side context propagation fix.

Checklist

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

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