* feat(chat): read one saved row per subagent
Adds the singular `subagent` journal block (one row per child, revised in
place, with a plain-text twin for older clients) and teaches every reader
to draw it: desktop and phone fold a turn's sibling rows into the existing
"Ran N subagents" row, the client and off-page rosters name children from
their own rows, the reopen pass settles a row a dead host left working,
and the phone and worker-transcript wires bound it. No producer writes the
block yet; that switch lands in the follow-up.
* fix(chat): open subagent background flag, separate nested group ids, page window skips subagent rows
- background is an open boolean so a later build's value never fails a chat load
- a helper's own helpers fold under a groupId of their own, so the phone opens them apart
- each subagent's own row rides along with the page window instead of counting as conversation
* refactor(chat): a subagent's group entry as one object
