Branching a session with linked child sessions consistently fails #5333

What happened

Creating a branch from this session fails with a generic “Operation failed” toast. The runtime host reports that ordinary branches cannot share linked child Session ownership with their source. This prevents branching a conversation after linked child sessions have been used.

How to reproduce

1. Use Maka Desktop and work in a session that has linked child sessions.
2. Attempt to branch from a turn in that session.
3. Observe the toast: “操作失败 / 任务操作失败，请稍后重试。”

Observed repeatedly at 2026-09-15 03:22–03:33 local time.

Expected: The branch is created, or the UI clearly explains any unsupported case and provides a recovery path.

Environment

• Maka version: 0.2.0-dev.31.20260913 (nightly, packaged)
• OS: macOS darwin 24.6.0 (arm64)
• Surface: Desktop
• Electron: 43.4.1
• Chrome: 150.0.7871.224
• Node.js: 24.18.1
• Locale: zh-CN
• Runtime Host protocol: v0, compatibility 151

Logs, screenshots, or additional context

Relevant main-process errors:

RuntimeHostOperationError: Ordinary branches cannot share linked child Session ownership with their source
  operation: session.branch.create
  code: operation_unavailable

RuntimeHostOperationError: Source Session has an active Turn
  operation: session.branch.create
  code: session_busy

The first error occurred repeatedly, including after retrying. Runtime Host was ready (1 connection, 3 operations, 0 residencies), and no local Runtime Host process exits were recorded. Full diagnostic report is available on request.