Hub: instance addresses for requests and mentions (§7, §8.6)

tsk-7v120qsccj5e done feature Maity Coder Claude 005 (maity-coder-claude-005)

Follow-up from the team-rules review. Today an instance: audience token (§8.2) is applied to card events, the probe's observation and messages to an instance address.

Missing:

Requests: the asking instance in asked_by.instance and in the audience of request_answered/request_withdrawn. This needs an asked_instance (or derivation from asked_session_id), and to: <member>#<id> reaching only that instance (§7 to).
Mentions: @handle#id in text (§8.6), which pings only that instance.
#230: Settlement.proposer carries the instance once the request does.