Add date range support to todo_list MCP tool
Extend todo_list() with optional day_to parameter for querying todos
across a date range. Single-day behavior unchanged when day_to is None.
- Range queries return markdown grouped by day with ### headers
- Validates date formats and range order with helpful error messages
- Same-day range (day == day_to) returns normal single-day format
- Add get_todo_days_in_range() helper to find todo files in range
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>