Issue #3622737: Add an attach-entity task for content a run computed, and drop startForEntity()

Adds an attach_entity task to orchestra_content. It binds the entity a process variable names, with the entity type fixed in the node's own configuration so the variable chooses only which entity of it, and under an attachment key so the binding can take a role of its own. An outcome variable makes a binding that cannot be made routable, set to success or failure so a flow condition can branch to an error path; left empty the failure propagates and the engine dead-letters it, because advancing would walk the run into a later step whose attachment is not there. The binding a run starts with stays on AttachmentManager::startForEntity(), which makes it inside the start transaction and so before the first node; this task is for what a run learns about itself while it runs.