A Claude Code workgroup peer (Diff, Code Review) or split/tab could end
up with "Close Sessions On End" as its end action even though spawn code
overrides it to the default: the override only lived in the in-memory
profile, and a later profile sync reconciles non-overridden keys back to
the shared profile's value. When that happened, the member auto-closed
the moment its command exited (e.g. a Diff pane whose difftool finished),
and closing a member session tears the whole workgroup down.

Add PTYSession.forceDefaultEndAction, honored by -[PTYSession endAction]
so it always reports the default regardless of the profile or a later
setEndAction. Set it on every spawned or adopted workgroup member (main
and nested peers via makeWorkgroupPeer, splits/tabs via the spawner,
restored members via registerNonPeer and adopt). The leader is left
alone so it still follows its profile and its termination still exits the
workgroup.