feat: maintain ascending date order in append_to_markdown tool

[bcosynot]

• Added _find_date_insertion_position() helper function to determine correct
  position for new date headings based on chronological order (oldest first)
• Updated append_to_markdown() to insert date headings in ascending order
  instead of always appending at the end
• Dates are now maintained in chronological sequence regardless of insertion order
• Added comprehensive test coverage for date ordering functionality:
  • Test adding older dates before newer dates
  • Test adding newer dates after older dates
  • Test inserting dates in the middle of existing sequences
  • Test maintaining order when adding multiple out-of-order dates
  • Test date ordering across months and years
  • Test handling non-date headings

This ensures the markdown worklog always has dates in ascending chronological
order, making it easier to navigate and maintain.