feat: add reversible workspace panels

docs/UX_VERTICAL_SLICE.md

@@ -19,6 +19,11 @@ Remaining visible gaps are terminal text selection and mouse modes, real
 decision-shelf actions, usage-detail provenance, and end-to-end platform
 accessibility/IME.
 
+Panel lifecycle is now visible in the slice: every panel has a Detach action,
+the workspace bar has Add panel, and the sidebar exposes Detached sessions for
+reattachment. The wording is deliberate because panel removal keeps its session
+alive; termination is not exposed as a visually equivalent close action.
+
 Screenshots for the audit are stored outside Git under
 `~/shots/2026-08/lumbridge-ui-audit/`. Accessibility and IME correctness cannot
 be established from screenshots and remain explicit runtime gates.
@@ -82,6 +87,8 @@ surface rather than a hosted Lumbridge control plane.
   leaving ordinary terminal arrows and text available to the PTY.
 - `Alt+1` through `Alt+6`: focus a pane directly while leaving terminal digits
   available to the PTY.
+- `Alt+Shift+N`: add or reattach the next panel in the deterministic slice.
+- `Alt+Shift+W`: detach the selected panel while its session keeps running.
 - `Cmd+K` on macOS or `Ctrl+K` on Linux: open the command palette.
 - `Shift+PageUp/PageDown`: move the selected live terminal through retained
   history; `Shift+Home/End` jumps to the history top/live bottom.