The matrix wake body description said '[matrix] <sender> in <room>:
<first-100c>...' (old single-line teaser). After feat(#1137) it's now
a richer format:
- Single room, 1 message: terse one-liner with sender + body
- Single room, N messages: count + room name
- Multi-room: bulleted list with per-room summaries
Also note that the same breakdown appears in get_loose_ends via the
UnreadMatrix entry.
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