feat(workspace): session view console as tabs, with New terminal

The 1/2/4 split view becomes a tab bar (ConsoleTabs.tsx): one tab per live
tmux slot (0-3) of the workspace, the active one being ?run=, every live tab's
pane kept mounted and hidden with CSS. The + menu opens a new tab in the first
free slot: New terminal (a plain shell - shell: true on POST
/api/lanes/:id/start, spawnShell, tool 'shell', no run_id/status change), any
installed enabled AI tool started bare, or Settings -> Agents. A tab's x stops
its session. splitViewStorage, &pane= and the sidebar's Ctrl/Cmd+click are gone.

client/src/components/__tests__/WorkspaceTree.test.tsx

@@ -238,11 +238,6 @@ describe("WorkspaceTree", () => {
     await waitFor(() => expect(live).toHaveAttribute("data-live", "true"));
     await user.click(live);
     expect(screen.getByTestId("where").textContent).toBe("/workspace?lane=1&run=agenthub-lane-1-2");
-    // Ctrl+click asks for a second pane.
-    fireEvent.click(live, { ctrlKey: true });
-    expect(screen.getByTestId("where").textContent).toBe(
-      "/workspace?lane=1&run=agenthub-lane-1-2&pane=1"
-    );
     await user.click(screen.getByTestId("workspace-session-s-old"));
     expect(screen.getByTestId("where").textContent).toBe("/workspace?lane=1&session=s-old");
     expect(api.lanes.action).not.toHaveBeenCalled();