Remove duplicate CompactionItem from RunItem union

[KanchiShimono]

Summary

• remove duplicated CompactionItem entry from RunItem union in src/agents/items.py
• keep a single CompactionItem entry while preserving intended member set
• no runtime behavior change; this is a type alias cleanup

Test plan

• make format
• make lint
• make typecheck

Issue number

• N/A

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

Thank you!