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/WorkspaceSessionList.tsx

@@ -8,8 +8,8 @@
  * renames it and its hover button archives it (`PATCH {name}` / `{archived}`,
  * which deletes nothing). A session with a LIVE pane (a `GET /api/run` handle
  * carrying its session id) opens that tmux pane on the Workspace page
- * (`?lane=<id>&run=<runId>`, Ctrl/Cmd+click `&pane=1` to open it in a second
- * split pane) — attaching, never starting or resuming anything;
+ * (`?lane=<id>&run=<runId>`, its tab in the session view) — attaching, never
+ * starting or resuming anything;
  * any other session opens in its workspace too (`?lane=<id>&session=<id>`,
  * `SessionReplay`): its conversation with a prompt box, resumed only when a
  * message is SENT, because a resume re-reads the whole transcript into context.
@@ -314,13 +314,10 @@ export function WorkspaceSessionList({
                 type="button"
                 data-testid={`workspace-session-${s.id}`}
                 data-live={run ? "true" : undefined}
-                onClick={(e) =>
+                onClick={() =>
                   navigate(
                     run
-                      ? // Ctrl/Cmd+click opens it beside the current pane.
-                        `/workspace?lane=${lane.id}&run=${encodeURIComponent(run.id)}${
-                          e.ctrlKey || e.metaKey ? "&pane=1" : ""
-                        }`
+                      ? `/workspace?lane=${lane.id}&run=${encodeURIComponent(run.id)}`
                       : `/workspace?lane=${lane.id}&session=${encodeURIComponent(s.id)}`
                   )
                 }