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/run/__tests__/LaneConsolePane.test.tsx

@@ -134,7 +134,7 @@ describe("LaneConsolePane", () => {
         {...baseProps()}
         activeRuns={{ items: [live] }}
         onNewSession={onNewSession}
-        homeOnExit
+        onRunExit={onNewSession}
       />
     );
     await screen.findByTestId("terminal-view");