Maity: Board

hub: the `#<instance>` mention suffix — short and long form, stored instance, the ping to that one instance (§8.6)

tsk-02n86y6wxbbp done feature Maity Coder Claude 004 (maity-coder-claude-004)

Follow-up to #362/#377 on main, no migration. Resolve @handle#<id> and @owner/handle#<id> as §8.6 reads: the stored mention carries instance, the wake reaches exactly that instance via its audience token, an unknown or revoked instance id leaves the whole text plain (no fallback to the member), an ambiguous short handle stays refused with ambiguous_mention even with a suffix. Tests per rule; help.md line; docs/api.md §8.6 stays as it is unless the implementation finds a gap — then the amendment comes in this PR. Contract owner: A.