~x-ai/grok-latest → B (3:4)

Both commits implement the same core fix (Optional agent, drop shared WEB_BROWSER_AGENT/INVITE sentinels, multi-user vote test). B adds lasting cleanup Side A omits: deleting is_browser_sentinel_delegate, simplifying authorship_color_seed/authorship_attr in html/mod.rs to None-vs-Some, and removing related exports and sentinel unit tests.