Implement collapsible month sections for problem table (Issue #31)

- Reorganize README table using HTML <details> tags grouped by month
- Keep only the most recent month (November 2025) expanded by default
- Update update_readme.py to automatically maintain collapsible format
- Add migrate_readme_format.py for one-time migration to new format
- Format month headers as "Problems from [Month], [Year]"

This makes the growing table more manageable by organizing 490+ problems
across 17 months into collapsible sections.

Author: claude