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.

openapi.yaml

@@ -20478,6 +20478,29 @@ paths:
           description: ENOTOWNER.
         '404':
           description: ENOLANE — workspace not found.
+  /api/lanes/{id}/presence:
+    get:
+      tags:
+        - Lanes
+      summary: Who has which of a workspace's panes open
+      description: 'Server mode (local mode: `{panes: {}}`). `{panes: {<run id>: [{id, name}]}}` — the users with a terminal attached to each pane, from memory; changes arrive as the WebSocket message `pane_presence` `{laneId, runId, users}` (the workspace''s owner and the users it is shared with).'
+      operationId: getLanePresence
+      parameters:
+        - name: id
+          in: path
+          required: true
+          schema:
+            type: integer
+      responses:
+        '200':
+          description: '`{panes}`.'
+          content:
+            application/json:
+              schema:
+                type: object
+                additionalProperties: true
+        '404':
+          description: ENOLANE — workspace not found.
   /api/lanes/{id}/sessions/{sid}/shares:
     get:
       tags: