Spec: agent-hive-group v3 -- seen/done reaction, standing briefs, deleted topic, /summary #6563

Adds the remaining laws from the competitor round of gHashTag/999-multibots-telegraf#3641 to specs/automation/agent-hive-group.t27 (v2 was #6558):

• the reaction is a courtesy: the answer counts whether or not the chat accepts reactions (999#3647);
• /brief: the topic brief wins over the group brief, at most 1000 chars, set by an admin only (999#3650);
• a topic a person deleted is recreated, at most once in 10 min (999#3652);
• /summary: 50 messages by default, capped by a 200-message log (999#3653).

Additive only: v1 and v2 laws are unchanged.