## Why
Command Center retains tasks across backend switches. Opening a retained task must use its owning backend, and restoring its draft must account for history or settings that changed while another backend was displayed.
## What changed
- Track each thread's first observed backend, reject conflicting ownership, and switch to the owning backend when opening a retained task. Require users to open tasks on other backends before running task actions.
- Keep discovery cursors, initialization, and pins per backend. Scope refreshes and usage requests to their owner, and handle background archive and delete notifications without consuming the displayed backend's pagination budget.
- Verify cached blank sessions are still idle and have no turns before reusing them, with a history-read fallback for older servers. Otherwise, resume saved history and preserve current server collaboration settings when restoring drafts.
- Use only interactive task-list pages for cloud discovery, which does not support the additional source filters.
## Testing
Add regression coverage for ownership conflicts, action routing, pins across backends, background removals, usage isolation, cloud discovery, and draft restoration with blank or populated history, including older-server fallbacks.
GitOrigin-RevId: 0d0913ec38dc16a839823008a4a84aa46ad8e6c6
Benjamin Carlsson
27294165dc