feat(server-mode): pane presence and typing audit (sharing phase 4)

A pane's tab shows the other people who have it open (pane-presence.js,
in memory, fed by the /ws-pty attach; GET /api/lanes/:id/presence and the
pane_presence broadcast to the owner and full collaborators) and says
"Billed to <name>" for a pane a collaborator opened (startedBy on the run).
The first real keystroke of each attach into a shared workspace's pane by
anyone but its owner is audited as `type into <run id>` (mouse, focus and
terminal replies to tmux do not count). The pty socket now pings, so a
vanished browser is ended at the next re-check and leaves the presence list.

server/lib/scope.js

@@ -514,6 +514,7 @@ function broadcastAudience(type, data) {
         owners: laneOwners(d.lane?.id),
         viewers: laneViewers(d.lane?.id, { sessions: true }),
       };
+    case "pane_presence":
     case "lane_hook_output":
     case "lane_hook_result":
     case "lane_runtime":