feat: add reversible workspace panels

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.