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.

docs/API.md

@@ -2126,6 +2126,17 @@ answers only that workspace's reads, the workspace list (filtered to it), its
 stage / up / down / hook / feature / mcp-sync writes and `/api/health`
 (`403 EPANESCOPE` — never a start or resume).
 
+```http
+GET /api/lanes/:id/presence              → { panes: { "<run id>": [{id, name}] } }
+```
+
+Who has which pane open (server mode; local mode `{panes: {}}`), in memory;
+changes come as the WebSocket message `pane_presence` `{laneId, runId, users}`
+to the workspace's owner and full collaborators. A run handle
+(`GET /api/run`) carries `startedBy: {id, name}` when a collaborator opened
+it — they pay for it. The first keystroke of a terminal attach into a shared
+workspace's pane by anyone but its owner is audited (`type into <run id>`).
+
 ```http
 GET /api/lanes/:id/sessions/:sid/shares  → { shares, canManage }
 PUT /api/lanes/:id/sessions/:sid/shares  {"userIds": [3]}   → { shares }