fix(gateway): announce declined 'all' scope in /sessions and /resume listings

A non-admin '/sessions all' or '/resume --all' silently downgraded to
chat-scoped listing with zero feedback, which reads as 'my session
vanished' (community Telegram report). Both surfaces now append a notice
that cross-chat listing requires a configured admin. Follows up the
salvaged current-session '(current)' marker (PR #68556, fixes #68547):
sibling tests updated to pin the new contract, new i18n key
gateway.resume.all_requires_admin added to all 17 locales, docs updated.

locales/en.yaml

@@ -281,6 +281,7 @@ gateway:
     matrix_cross_room_success: "⚠️ Cross-room resume: resumed **{title}** inside Matrix room **{room}**.\nFuture messages in this room will use that transcript until `/reset` or another `/resume`.{msg_part}"
     blocked_not_owner:     "⚠️ /resume blocked: '**{name}**' belongs to a different user or chat. You can only resume sessions from this chat."
     no_named_sessions:     "No named sessions found.\nUse `/title My Session` to name your current session, then `/resume My Session` to return to it later."
+    all_requires_admin: "_Note: `--all` (cross-chat listing) requires a configured admin; showing this chat's sessions only._"
     list_header:           "📋 **Named Sessions**\n"
     list_item:             "• **{title}**{preview_part}"
     list_item_numbered:    "{index}. **{title}**{preview_part}"