Link planning and implementation threads when using Implement in new thread #978

Problem

When using Implement in new thread, the original planning thread remains in a Plan Ready state. That feels clunky and unintuitive because the work has effectively continued elsewhere, but the original thread does not clearly indicate that relationship.

Desired behavior

The planning thread and the new implementation thread should be explicitly linked in the UI so it is obvious that the plan was handed off rather than abandoned.

Possible directions:

• Show that the Plan Ready thread spawned a linked implementation thread
• Add a backlink from the implementation thread to its source planning thread
• Replace or augment the Plan Ready state with a clearer handoff/continued-elsewhere state

Why this matters

Right now the old thread can look stale or incomplete even though the user intentionally continued execution in a new thread. That makes thread history harder to understand and adds friction when scanning conversations later.

Acceptance criteria

• Using Implement in new thread creates a visible relationship between the source planning thread and the new implementation thread
• The original thread no longer feels stranded in a misleading Plan Ready state
• Users can navigate between the planning thread and the spawned implementation thread
• The UI communicates whether the plan was continued, handed off, or superseded by the new thread