A2A timeline: notices and platform prompts on web (design groups 4 and 5) #698

What

Build groups 4 and 5 of the agreed A2A timeline design on web (design: project artifact upstream-merge-2026-10-24/a2a-timeline-design.md, settled with Jackson on 2026-10-24).

• Group 4, platform notices. One grey line on the left with an icon, a plain summary, the time and a chevron; opening it shows the detail. It covers:
  • J5's notices: silence, not delivered, withdrawn or dropped Exchanges, removed queued messages;
  • J5's Crew gate and seat-finished notices, demoted from cards on purpose;
  • upstream's own platform messages: restart and provider continuations, PR-watch updates, delegated and background task completions.
• Group 5, platform prompts. An outlined pill on the left: automation runs (upstream's Automation row, moved left with its fill removed), playbook steps handed to a seat, native questions to a placement parent, delegated-task questions, idle compaction.
• The "Raw A2A delivery" card goes away; an unrecognised notice falls back to upstream's row.

Shown raw today

Three J5 notices have no card, so no parser reads them and the timeline shows the raw "Raw A2A delivery" envelope: message not delivered, a peer dropped the exchange, and the ask was withdrawn. j5/main and the v0.1.3 advance (#705) both draw them this way. They are the first notices to move to group 4.

Why it isn't in the advance

The 2026-10-24 advance PR builds the shared classifier and groups 1–3 (the person, other people, other participants), which is the response to upstream's fold (pingdotgg#23135). Groups 4 and 5 restyle messages upstream writes, which is a new product divergence rather than a reaction to the advance. Jackson also expects to redo or retire the Crew cards. Until this lands, J5's notices keep today's cards (the three above stay raw) and upstream's platform messages keep upstream's look.

Done when

• Groups 4 and 5 render as the design describes, from the shared classifier in packages/client-runtime/src/j5/.
• The register entry that replaced D16 covers the restyled upstream messages, and docs/j5/product/features/thread-a2a-rendering.md describes all five groups.
• FORK.md records the hunks (about 1–2 for group 5 in TimelineUserMessageRow.tsx, plus a class edit on upstream's AutomationMessageRow).
• Before/after screenshots of each notice and prompt kind.