Auto-assign an owner to blocked follow-up tasks on creation

[kai-linux]

Goal

Ensure blocked-task follow-ups are created with a concrete owner so they do not stall in an unassigned state after escalation or prompt inspection.

Success Criteria

• Newly created follow-up tasks from blocked outcomes always include an assigned agent or routing target.
• The assignment happens at follow-up creation time, not in a later manual pass.
• Blocked follow-ups no longer default to an empty or none agent field.

Constraints

• Prefer minimal diffs