Repository: the guest rule in AGENTS.md

Internal: AGENTS.md, which every coding session reads first, says that a team's people include guests and how a query on team_members treats them: TeamAreaAccess::memberFilterSql() for lists and pickers, guestNeedsAreaSql() for reads of records, and role <> 'guest' for a check or count that means the team's members. The rule was only in docs/TEAM_GUESTS.md before.