_session_source_for_agent now drops an inherited tui/desktop
HERMES_SESSION_SOURCE for finite chats, but main.py exports the
documented --source flag through the very same variable, so
'hermes chat -q --source tui' was silently relabelled 'cli' with no way
to opt out. main.py marks an explicit flag with
HERMES_SESSION_SOURCE_EXPLICIT=1 and the resolver keeps the label when
that marker is set; no internal launcher passes --source tui/desktop, so
inherited transport labels are still dropped.
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