feat: add reversible workspace panels

docs/TESTING.md

@@ -104,7 +104,8 @@ who already have the final cargo-watch release installed.
 ## Implemented vertical-slice coverage
 
 - The framework-neutral six-surface model has deterministic tests for focus,
-  direct selection, needs-input transitions, command-palette lifecycle,
+  direct selection, reversible attach/detach, last-panel protection, detached
+  background updates, needs-input transitions, command-palette lifecycle,
   identical GPUI/Floem action replay, bounded output, and workload counters.
 - `lumbridge-pty` runs synthetic `/bin/sh` tests for raw output, non-zero exits,
   input, resize, one-chunk backpressure, hung-process termination, invalid
@@ -117,7 +118,8 @@ who already have the final cargo-watch release installed.
   alternate screen, bracketed paste, protocol replies, sanitized title and
   blocked OSC 52 behavior, key encoding, application-cursor mode, and resize.
 - `lumbridge-core` tests capability-gated split/select/rename/close commands,
-  idempotent request IDs, close-tree promotion, and atomic agentic setup plans.
+  idempotent request IDs, close-tree promotion, detached-pane reattachment,
+  Execute-gated detached termination, and atomic agentic setup plans.
 - The GPUI slice tests its key-event adapter, responsive one/three/five-panel
   geometry, selected-panel windowing, per-panel 20/60/20 PTY sizing, xterm
   256-color conversion, styled-run coalescing, cursor-run boundaries, and