feat(ui): call lanes "workspaces" in the UI, CLI output and docs

Copy-only rename (workspace redesign, phase 0). The word a user reads
changes — "Lane" → "Workspace", "Add lane" → "Add workspace" — in both
locales, in agenthub CLI prose and help, in the server's error messages
and in the docs. Every identifier stays: /api/lanes, `agenthub lanes`,
--lane, AGENTHUB_LANE_ID, error codes, tmux names, DB tables, file,
component and i18n key names. docs/LANES.md keeps its name, retitled
"Workspaces (lanes)".

client/src/pages/__tests__/Workspace.test.tsx

@@ -499,7 +499,9 @@ describe("Workspace layout", () => {
     expect(strip.firstElementChild).toBe(screen.getByTestId("lane-tile-add"));
 
     await user.click(screen.getByTestId("lane-tile-add"));
-    expect(await screen.findByText("Create a lane from a working directory")).toBeInTheDocument();
+    expect(
+      await screen.findByText("Create a workspace from a working directory")
+    ).toBeInTheDocument();
   });
 
   it("gives every lane a tile in the carousel", async () => {