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/pages/__tests__/Workspace.changesPanel.test.tsx

@@ -59,6 +59,7 @@ vi.mock("../../components/lanes/WorkspaceHome", () => ({
   WorkspaceHome: () => <div data-testid="workspace-home" />,
   paneRunId: (id: number, slot: number) =>
     slot ? `agenthub-lane-${id}-${slot}` : `agenthub-lane-${id}`,
+  freeSlot: () => 0,
 }));
 vi.mock("../../lib/api", async (importOriginal) => {
   const actual = await importOriginal<Record<string, unknown>>();