Make the workspace right pane a first-class tab host (#3287)

* feat(app): add persistent workspace side panes

Keep the common review and file-browsing workflow beside the active agent without tearing down pane contents. Workspace tabs now share one compact chip language, assistant file links route into the retained right pane, and diff/file trees share stable geometry.

* fix(app): localize pull request empty state

* test(app): update explorer pane coverage

* fix(app): preserve explorer pane workflows

* test(app): make explorer tab selection idempotent

* fix(app): persist workspace review pins

* fix(app): preserve sidebar scrim drag hook

* fix(app): restore companion pane interactions

* test(app): follow selected sidebar surface

packages/app/src/utils/split-navigation.ts

@@ -171,6 +171,9 @@ function collectPaneBounds(
   bounds: { left: number; top: number; right: number; bottom: number },
 ): PaneBounds[] {
   if (node.kind === "pane") {
+    if (node.pane.hidden === true) {
+      return [];
+    }
     return [
       {
         paneId: node.pane.id,