Grid view subagent pills are dead code — listener signature (event, data) doesn't match preload's single-arg callback

[JeanBaptisteRenard]

Summary

Found while implementing #129 (PR #130): grid-view.js's initSubagentListeners() registers callbacks as (event, data) => {...}, but preload.js invokes subagent listeners with a single payload argument. data is therefore always undefined in grid-view, its activeSubagents Map never populates, and the grid-card subagent pills never render.

Impact

The grid view silently lost its live-subagent display. No error is thrown — the feature just never fires.

Fix

Align the callback signature with the single-arg pattern used by jsonl-viewer.js (and now sidebar.js in PR #130), and add a test that drives the callbacks through the same dom-setup.js emit helpers introduced in #130.

Refs #129.