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/components/lanes/__tests__/LaneChangesPanel.test.tsx

@@ -207,7 +207,7 @@ describe("LaneChangesPanel", () => {
     });
     render(<LaneChangesPanel laneId={7} />);
     expect(
-      await screen.findByText("This lane's directory is not a readable git repository.")
+      await screen.findByText("This workspace's directory is not a readable git repository.")
     ).toBeInTheDocument();
   });