Problem: need persistent task storage with forward-compatible
schema and unknown field preservation.
Solution: add store module with load/save, add/update/delete,
ID allocation, and UDA round-trip preservation. Data stored at
stdpath('data')/todo/tasks.json with version 1 schema.
Problem: need user-configurable settings with sensible defaults.
Solution: add config module that merges vim.g.todo with defaults
for data_path, default_view, default_category, date_format, and
date_syntax.
