7 tasks: pure divider/picker helpers, showSplitButton header wiring,
split-container CSS, SplitTerminalPane (Pane B's independent xterm+WS),
open/close orchestration with picker and divider drag, auto-collapse on
either session ending, and an architecture-invariants entry.
Also folds in the "detach session" prior art discovered mid-brainstorm
into the spec's architecture section.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
