Add bulk cleanup for unused workflow contracts

[migmartri]

Problem

Workflow contracts can become orphaned over time — created but never associated with a workflow, or left behind after their workflows are deleted. There is currently no way to bulk-clean these up; operators must delete them one by one.

Proposed Solution

Add a PurgeUnused RPC and CLI command to bulk-delete workflow contracts that have no associated workflows. The existing List already returns workflow_refs per contract, so clients can preview which contracts are unused before purging.

Implementation

• PR: feat(api): add PurgeUnused RPC for bulk-deleting unused workflow contracts #3071