fix(slack): resolve a cold channel's workspace from the sole authenticated team

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.

plugins/platforms/slack/adapter.py

@@ -1909,8 +1909,14 @@ class SlackAdapter(BasePlatformAdapter):
     def scope_id_for_chat(self, chat_id: str) -> Optional[str]:
         """Return the workspace id owning ``chat_id``.
         ``None`` for unknown channels and for channels claimed by several workspaces (dropped from
-        the map) — no scope beats a wrong one."""
+        the map) — no scope beats a wrong one. A channel unseen since boot (the map only fills from
+        inbound events) still resolves when exactly one workspace is authenticated: every inbound
+        reply will carry that team_id, so a caller keying a session on it must match (#111896)."""
         team_id = chat_id and (getattr(self, "_channel_team", None) or {}).get(str(chat_id))
+        if not team_id and chat_id and str(chat_id) not in (getattr(self, "_channel_teams", None) or {}):
+            team_clients = getattr(self, "_team_clients", None) or {}
+            if len(team_clients) == 1:
+                team_id = next(iter(team_clients))
         return str(team_id) if team_id else None
 
     def _get_client(self, chat_id: str, team_id: Optional[str] = None) -> Any: