Teammate Pane Backend Removed

The InProcessBackend and PaneBackendExecutor classes (which backed the teammate/swarm pane UI) have been removed from the codebase. Associated log messages ([InProcessBackend] ..., [PaneBackendExecutor] ..., [BackendRegistry] ...), the --teammate-mode CLI flag, and the ITermBackend pane show/hide/rebalance stubs are gone.

Users relying on the teammate swarm UI should be aware this infrastructure has been cleaned out. The teammateMode configuration key still exists for compatibility, but the pane-spawning backend is no longer present.