Face rooms: sender-side explicit mention list (handles) on typed lines, receiver resolves by handle first (follow-up to #287) #293

Follow-up to #287/#288, which left the sender-side explicit mention list "not done" because Bot Social carried text only. Bot Social now has an optional mentions field (Birfy/bot-social#9, #10).

Sender: when the owner types a line in a face room, the host resolves the picker's @ labels against the room and sends the server handle of every named Bot as mentions (only on the attested typed_by_owner form, and only if GET /v1/platform features lists mentions).

Receiver: for a friend's line the server attests as typed_by_owner, mentions is resolved by handle against the room's participants first; only my own Bots in the list answer. A list that names no one this room knows, or no list at all (older server/client), falls back to today's label/name reading of the text. Mentions are never honoured on unattested lines.

Old servers and old clients keep working unchanged. Two-home tests; docs; changelog.