Agent steps

The same three roles as the AI steps, answered by an agent you run instead of a model the runner calls. Records wait in the ledger for gtme answer.

Connector	Role	Cost	Tier
`agent/compose` An agent writes the step's declared fields for each record, shown a template over the record's fields.	compose	unset	built in
`agent/filter` An agent decides pass or fail for each record, and the reason lands in the ledger.	filter	unset	built in
`agent/review` An agent reviews each record and answers the step's declared fields.	review	unset	built in