fix(run): turn on tmux mouse mode on every attach, not just at session creation

Sessions created before the mouse-on fix landed (or before the server
restarted to pick it up) never got the set-option call, so their pane
still had no drag/wheel scroll. Moving it into pty-attach's attach()
makes it idempotent per-connection instead of once-at-birth.

server/lib/tmux.js

@@ -40,10 +40,17 @@ function hasSession(name) {
  */
 function newSession({ name, cwd, argv }) {
   execImpl(["new-session", "-d", "-s", name, "-c", cwd, "--", ...argv]);
-  // Without this the pane has no scrollbar/drag scroll at all — wheel events
-  // just pass through to the running program instead of entering tmux's own
-  // copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
-  // automatically once mouse mode is on, so no client-side change is needed.
+  enableMouse(name);
+}
+
+// Without this the pane has no scrollbar/drag scroll at all — wheel events
+// just pass through to the running program instead of entering tmux's own
+// copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
+// automatically once mouse mode is on, so no client-side change is needed.
+// Called on every attach (not just at creation) so a session started before
+// this option existed — or before a server restart picked up the change —
+// still gets it; `set-option` is idempotent, so re-running it is harmless.
+function enableMouse(name) {
   execImpl(["set-option", "-t", name, "mouse", "on"]);
 }
 
@@ -112,6 +119,7 @@ function isTmuxAvailable() {
 module.exports = {
   hasSession,
   newSession,
+  enableMouse,
   paneCommand,
   sendCommand,
   killSession,