消息中心:重复通知的分组/去重/折叠(scheduled digest 每分钟一条把收件箱刷屏) #2765

Problem

The message center lists every notification individually with no grouping. In the showcase, showcase_scheduled_digest (interval trigger) fires ~once a minute, so the inbox fills with 10+ identical "Scheduled project digest" entries within minutes — unread climbed to 20, the bell badge shows 9+. The signal-to-noise is poor and a real workspace with any recurring notification would hit the same wall.

Found in the #3358 sweep (§7).

Ask

• Group / collapse identical or same-topic notifications in the message center (e.g. Scheduled project digest ×10, expandable), instead of one row each.
• Dedup / coalesce repeats of the same (topic, title) within a window.
• Per-group affordance: "mark all of this type read", and count the group as one unread for the badge (or a smarter badge that reflects distinct topics).

Companion (framework/showcase side, optional separate track): the showcase digest cadence (~1/min) is aggressive for a demo — dial it down so first-boot isn't a flood.

Why

Notification grouping is table-stakes for an inbox; without it, any recurring notification drowns the actionable ones (assignments, @mentions, approvals).

Refs: objectstack-ai/objectstack#3358 (sweep). Related console-polish umbrella: #2762.