[P3 adjudication] A-V4: extract service.Bus into a leaf package

[DeliciousBuding]

Audit P3 (A-V4). Measured: Bus defined in eventbus.go; agentteam imports Bus via parent service pkg (child->parent reverse edge). Changes export path service.Bus. Tier: PLAN. Recommended: move Bus to internal/bus leaf package; update importers. Owner: TBD. Milestone: refactor-backlog.