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/panels/files-panel.tsx

@@ -0,0 +1,65 @@
+import { Text, View } from "react-native";
+import { FolderTree } from "lucide-react-native";
+import { withUnistyles } from "react-native-unistyles";
+import { useCallback } from "react";
+import { useTranslation } from "react-i18next";
+import invariant from "tiny-invariant";
+import { FileExplorerPane } from "@/components/file-explorer-pane";
+import { usePaneContext } from "@/panels/pane-context";
+import type { PanelRegistration } from "@/panels/panel-registry";
+import { useAddFileToChat } from "@/panels/use-add-file-to-chat";
+import { useWorkspaceDirectory } from "@/stores/session-store-hooks";
+
+const ThemedFolderTree = withUnistyles(FolderTree);
+const CENTERED_PADDED_STYLE = {
+  flex: 1,
+  alignItems: "center",
+  justifyContent: "center",
+  padding: 16,
+} as const;
+
+function useFilesPanelDescriptor() {
+  const { t } = useTranslation();
+  return {
+    label: t("panels.files.label"),
+    subtitle: t("panels.files.subtitle"),
+    tooltip: t("panels.files.tooltip"),
+    titleState: "ready" as const,
+    icon: ThemedFolderTree,
+    statusBucket: null,
+  };
+}
+
+function FilesPanel() {
+  const { t } = useTranslation();
+  const { serverId, workspaceId, target, retargetCurrentTab } = usePaneContext();
+  const workspaceRoot = useWorkspaceDirectory(serverId, workspaceId);
+  const { addFile, canAddToChat } = useAddFileToChat({ serverId, workspaceId });
+  invariant(target.kind === "files", "FilesPanel requires files target");
+  const onOpenFile = useCallback(
+    (path: string) => retargetCurrentTab({ kind: "file", path }),
+    [retargetCurrentTab],
+  );
+  if (!workspaceRoot) {
+    return (
+      <View style={CENTERED_PADDED_STYLE}>
+        <Text>{t("panels.file.directoryMissing")}</Text>
+      </View>
+    );
+  }
+  return (
+    <FileExplorerPane
+      serverId={serverId}
+      workspaceId={workspaceId}
+      workspaceRoot={workspaceRoot}
+      onOpenFile={onOpenFile}
+      onAddToChat={canAddToChat ? addFile : undefined}
+    />
+  );
+}
+
+export const filesPanelRegistration: PanelRegistration<"files"> = {
+  kind: "files",
+  component: FilesPanel,
+  useDescriptor: useFilesPanelDescriptor,
+};