fix: repair monthly report workflow chain

[Pigbibi]

Summary

• pass the required --hourly-dir argument and ensure the monthly-review label exists before issue creation
• support manual AI review dispatch, allow Claude to read the target issue with MCP tools, and post/update the final review comment from the execution log
• add regression coverage for the workflow config and comment extraction helper

Verification

• python3 -m unittest tests/test_monthly_report_bundle.py tests/test_monthly_report_workflow_config.py tests/test_post_monthly_ai_review_comment.py
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/monthly_report.yml"); YAML.load_file(".github/workflows/ai_review.yml"); puts "yaml ok"'
• Monthly Execution Report run 23840033940 on fix/monthly-report-workflow succeeded
• AI Monthly Review run 23840811392 on fix/monthly-report-workflow succeeded and updated issue Monthly Execution Review: 2026-03 #9 comment