feat: multi-project workspaces and project window groups (#448)

Workspaces open several local project folders in one IDE session (File › New / Open / Manage Workspace…, or Attach when opening a project, which creates a workspace automatically for a plain project window). Member folders are the Project view roots and take part in indexing, Find in Files, Git, language servers, run configurations and builds; agent tasks choose which member they work in. Window › Merge All Project Windows groups project windows into one window with project tabs (switch, reorder, close, move to a new window), restored on reopen.

Closes #241
Closes #198
Closes #286

src/workbench/projectWindow.test.ts

@@ -0,0 +1,46 @@
+import { beforeEach, expect, it, vi } from "vitest";
+import { useProject } from "@/platform/project";
+import { chooseProjectWindow, openPolicy } from "./projectWindow";
+import { ask } from "./messages";
+import { commands } from "@/platform/ipc";
+
+vi.mock("./messages", () => ({ ask: vi.fn() }));
+vi.mock("@/platform/remote", () => ({
+  isRemoteWindow: () => false,
+  connectRemote: vi.fn(),
+}));
+vi.mock("@/platform/ipc", () => ({
+  call: (p: unknown) => p,
+  commands: {
+    settingsSet: vi.fn(async () => ({ general: { openProjectIn: "attach" } })),
+  },
+}));
+beforeEach(() => {
+  vi.clearAllMocks();
+  useProject.setState({
+    project: { root: "/a", name: "A", settings: {}, isGit: false },
+    settings: {},
+    lightEdit: false,
+  });
+});
+it("offers IDEA's four choices with This Window primary and remembers Attach", async () => {
+  vi.mocked(ask).mockResolvedValue({ button: "attach", checked: true });
+  expect(await chooseProjectWindow("B")).toBe("attach");
+  const prompt = vi.mocked(ask).mock.calls[0]![0];
+  expect(prompt.buttons.map((b) => b.id)).toEqual(["cancel", "attach", "new", "same"]);
+  expect(prompt.buttons.filter((b) => b.primary).map((b) => b.id)).toEqual(["same"]);
+  expect(commands.settingsSet).toHaveBeenCalledWith("global", "general.openProjectIn", '"attach"');
+  expect(openPolicy()).toBe("attach");
+  expect(await chooseProjectWindow("C")).toBe("attach");
+  expect(ask).toHaveBeenCalledTimes(1);
+});
+it("does not attach without a project or when opening a remote project", async () => {
+  useProject.setState({
+    settings: { general: { openProjectIn: "attach" } } as never,
+  });
+  vi.mocked(ask).mockResolvedValue({ button: "cancel", checked: false });
+  expect(await chooseProjectWindow("remote", false)).toBeNull();
+  expect(vi.mocked(ask).mock.calls[0]![0].buttons.map((b) => b.id)).not.toContain("attach");
+  useProject.setState({ project: null });
+  expect(await chooseProjectWindow("B")).toBe("same");
+});