implement editor-above-Claude split in workspace

Editor opens above Claude via Ctrl+E with a draggable divider at
35% height (clamped 15-70%). Ctrl+W or Escape closes it. The
workspace no longer renders a tab bar — Claude is the always-visible
primary surface per D-047/D-048; the editor is a split overlay
per D-049, not a tab. The _WorkspaceSlot separates editor from
primary content tabs and renders them as a vertical split when the
editor is open.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/lib/builtin/default_layout/src/extension.dart

@@ -81,6 +81,21 @@ class DefaultLayoutExtension extends ClideExtension {
           defaultBinding: 'escape',
           run: _exitFocusMode,
         ),
+        // Editor split (D-049, D-054)
+        CommandContribution(
+          id: 'editor.open',
+          command: 'editor.open',
+          title: 'Open Editor',
+          defaultBinding: 'ctrl+e',
+          run: _openEditor,
+        ),
+        CommandContribution(
+          id: 'editor.close',
+          command: 'editor.close',
+          title: 'Close Editor',
+          defaultBinding: 'ctrl+w',
+          run: _closeEditor,
+        ),
         // Sidebar section switching (D-054): alt+1 through alt+5
         for (var i = 0; i < 5; i++)
           CommandContribution(
@@ -182,6 +197,10 @@ class DefaultLayoutExtension extends ClideExtension {
       ctx.arrangement.exitFocusMode();
       return IpcResponse.ok(id: '', data: {'focusMode': false});
     }
+    if (ctx.arrangement.editorOpen) {
+      ctx.arrangement.closeEditor();
+      return IpcResponse.ok(id: '', data: {'editorOpen': false});
+    }
     if (ctx.palette.isOpen) {
       ctx.palette.toggle();
       return IpcResponse.ok(id: '', data: {'palette': false});
@@ -189,6 +208,23 @@ class DefaultLayoutExtension extends ClideExtension {
     return IpcResponse.ok(id: '', data: {});
   }
 
+  Future<IpcResponse> _openEditor(List<String> args) async {
+    final ctx = _ctx;
+    if (ctx == null) return _notActivated();
+    ctx.arrangement.openEditor();
+    return IpcResponse.ok(id: '', data: {'editorOpen': true});
+  }
+
+  Future<IpcResponse> _closeEditor(List<String> args) async {
+    final ctx = _ctx;
+    if (ctx == null) return _notActivated();
+    if (ctx.arrangement.editorOpen) {
+      ctx.arrangement.closeEditor();
+      return IpcResponse.ok(id: '', data: {'editorOpen': false});
+    }
+    return IpcResponse.ok(id: '', data: {});
+  }
+
   Future<IpcResponse> _switchSidebarSection(int index) async {
     final ctx = _ctx;
     if (ctx == null) return _notActivated();