fix: close stale monthly task issues

[Pigbibi]

Summary

• close repo-scoped monthly task issues when a new optimization plan no longer assigns any actions there
• keep the issue marker and planner link so the close-out is traceable
• add coverage for the closed-issue body rendering

Testing

• python3 -m unittest tests/test_build_monthly_optimization_plan.py tests/test_prepare_auto_optimization_pr.py tests/test_fanout_monthly_optimization_tasks.py tests/test_auto_optimization_pr_workflow_config.py
• uvx ruff check scripts tests