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/lanes/WorkspaceHome.tsx

@@ -2,7 +2,9 @@
  * @file WorkspaceHome — what `/workspace?lane=<id>` opens on, shaped like a
  * Claude Desktop project page: the name, a prompt box that starts a NEW
  * session in the first free pane slot (0–3, `POST /api/lanes/:id/start`) and
- * then opens it (`&run=<runId>`) — the ONE place a session is started. Inside
+ * then opens it (`&run=<runId>`) — the one place a session starts WITH a
+ * prompt (the session view's tab bar `+` only starts a bare CLI or a shell,
+ * `ConsoleTabs.tsx`). Inside
  * the box, like Claude Desktop's: AI (the session's own tool, the workspace's
  * by default) and one Model chip (`ModelMenu`: the tool's own list or the
  * router's models, plus Effort); under it the quieter choices — AI Router