Adds two new recipient types to support argument-driven assignees in
escalation channels. The assignee email or group name is resolved from
a named input argument at runtime, rather than being fixed at design time.

- ArgumentEmail (type=7): resolves to an email recipient
- ArgumentGroupName (type=8): resolves to a group-name recipient

Both carry argumentName (snake_case: argument_name) which is looked up
in the execution input at runtime by the escalation tool layer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>