Attach artifacts to an agent session

Attaches artifacts to an agent session without writing a message. Additive: each supplied artifact is merged into the session's related_artifacts under the same identity rule the message endpoints use — inline artifacts by name, remote artifacts by entity_kind + entity_id — with the supplied artifact winning on a collision and everything else left untouched. Re-sending the same artifact is therefore idempotent. Returns the session's merged artifact list, not the whole session. Use this, rather than the message endpoints, to record a link (a deployment event, a test run, another agent session) onto a session that is not otherwise being written to.