scope_id_for_chat only consulted the channel→team map, which is empty right after boot (and
after a reconnect) until an inbound event from that channel arrives. A /handoff into a Slack home
without a stored scope_id (SLACK_HOME_CHANNEL env homes, or config homes never re-set via
/sethome) therefore built a key without the team while every thread reply carries it — the
handed-off thread was still orphaned across a restart (#111896).
When the map has no entry and the channel is not known to be shared across workspaces, fall back
to the single authenticated workspace (filled by auth.test at connect); multi-workspace installs
keep returning None.
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