feat: add reversible workspace panels

docs/ARCHITECTURE.md

@@ -89,6 +89,12 @@ approval boundary. The selected panel alone owns keyboard input. Six surfaces
 continue updating so performance comparisons remain meaningful, with a sliding
 visible window keeping the selected pane onscreen. See decisions 0008 and 0009.
 
+Panel attachment is independent from session lifetime. Detaching removes a pane
+from the layout tree but retains its definition in the workspace's detached
+registry and leaves its runtime-owned process alive. Reattaching restores that
+identity without a launch. Terminating an attached or detached session is a
+separate Execute-capability operation. See decision 0010.
+
 We should evaluate, not blindly copy, WezTerm, Zellij, RMUX, tmux, and cmux. The
 first spike must compare a reusable terminal crate with a small first-party layer.
 Remaining correctness cases include OSC 8 links, Kitty

docs/PRODUCT_SPEC.md

@@ -79,6 +79,13 @@ capability each choice would need. Suggestions are inert data. Choosing one may
 prepare a typed workspace plan, but any command, file mutation, credential use,
 or external message still passes through the normal approval and command plane.
 
+Removing a panel detaches it from the layout; it does not terminate the pane's
+PTY, agent, browser, or remote session. Detached sessions remain visible in a
+workspace tray and can be reattached without relaunching. Termination is a
+separate, explicitly named Execute-capability action with confirmation. The add
+control creates the default panel beside the selection, with a menu for choosing
+a surface or reattaching a running session. At least one panel remains attached.
+
 ## Lumbridge Harness
 
 Lumbridge Harness is an optional orchestrator distributed as a separately

docs/RESEARCH.md

@@ -11,7 +11,8 @@ They are shallow snapshots for study, not dependencies or vendored source.
 ## Multiplexers and terminal engines
 
 - `manaflow-ai/cmux`: the current cmux product; native workspace/pane model,
-  automation socket, embedded browser, agent hooks, and terminal UX.
+  automation socket, embedded browser, agent hooks, and terminal UX. The
+  captured tree is GPL-3.0-or-later, so it is behavior/UX study only.
 - `Helvesec/rmux`: Rust multiplexer engine, daemon, typed SDKs, tmux surface.
 - `tmux/tmux`: the durable client/server and command model to remain compatible
   with where useful.

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

docs/UI_SPIKE_SCORECARD.md

@@ -25,8 +25,11 @@ The current programs establish dependency, build, launch, and interaction
 baselines. GPUI now routes a real PTY through a VT engine and keyboard encoder,
 paints coalesced styled cell runs and cursor shapes, and derives PTY rows/columns
 from the 60% terminal region inside each responsive one/three/five-panel layout
-when the window changes. Retained-history page/top/bottom navigation is wired. Text selection,
-mouse modes, a native Markdown editor, and one isolated browser child remain.
+when the window or attached panel count changes. Retained-history
+page/top/bottom navigation is wired. Panel detach/reattach keeps background
+surface updates alive and exposes detached sessions in the sidebar. Text
+selection, mouse modes, a native Markdown editor, and one isolated browser child
+remain.
 
 ## Ubuntu baseline — metal, 2026-08-31
 

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.

docs/decisions/0006-workspace-command-plane.md

@@ -14,6 +14,11 @@ process requires Execute; layout-only changes require Configure. Launch intents
 refer to a user shell or an approved harness profile and working directory, not
 arbitrary executable arguments or secret values.
 
+Detaching and reattaching an existing pane are layout-only Configure operations.
+Detached pane definitions remain outside the layout tree so reattachment does
+not imply a new launch. Terminating either an attached or detached session
+remains an Execute operation. See decision 0010.
+
 This reducer is not an authorization bypass. The future IPC layer authenticates
 the caller and supplies its granted capability; the reducer rechecks it before
 mutation. Durable audit events record request and object identities, never

docs/decisions/0010-panel-lifecycle-detach-before-terminate.md

@@ -0,0 +1,37 @@
+# 0010: Removing a panel detaches its session before termination
+
+Status: accepted for implementation.
+
+A workspace panel is a view attachment, not the lifetime owner of a PTY, agent,
+browser, or remote session. The normal remove action is therefore **Detach
+panel**. It removes the panel from the visible layout, keeps its stable pane and
+session identities, preserves output and decision state, and exposes the item in
+a Detached sessions tray. Reattaching it is a Configure-capability operation and
+must not launch a second process.
+
+Termination is a separate Execute-capability operation. It is presented as
+**Terminate session**, never as an ambiguous close icon, and will require a
+confirmation that names the process, target machine, and any dirty or waiting
+state. Detaching the selected panel chooses its next attached sibling, falling
+back to the previous sibling. A workspace retains at least one attached panel.
+
+The add affordance has two product paths: quick activation creates the user's
+default panel beside the selection, while its menu offers Terminal, Browser,
+Markdown, Review, and Reattach running session. The deterministic UI spike uses
+six fixed fixtures, starts with five attached, and reattaches the first detached
+fixture when Add panel is activated. That fixture limit is test scaffolding, not
+a product limit.
+
+The typed command plane retains detached pane definitions separately from the
+layout tree. `ClosePane(Detach)` moves a definition into that registry,
+`AttachPane` restores it without a launch intent, and `ClosePane(Terminate)`
+requires Execute authority whether the pane is attached or detached. The future
+runtime registry remains the process owner throughout these layout transitions.
+
+This interaction vocabulary was informed by behavior study of cmux at
+`fc36aea87e3b4152637597443d7aa7c22e4ec9f8` and Orca at
+`02a7742406a5a84fb372d6255d5a4367421990bd`. cmux is GPL-3.0-or-later and was
+used only to study workspace/split shortcuts and close-versus-detached process
+behavior. Orca is MIT-licensed and was used to study worktree-first navigation,
+durable terminal state, and mixed surface density. No implementation code or
+visual asset was copied from either project.