Add mypy type checking and fix type-safety issues - #30

- Wire mypy into the flake checks and dev shell; configure it in
  pyproject.toml with check_untyped_defs and scoped ignores for the
  untyped Google/xdg libraries.
- Parse --status into TaskStatus at the CLI boundary so invalid values
  fail with a usage error; GoogleApiService now takes TaskStatus | None.
- Fix Task/TaskList __eq__ signatures (other: object, NotImplemented)
  and document that id/position are deliberately ignored.
- Fix issues surfaced by mypy: guard the optional source map in the
  parser, annotate reconcile op dicts via TaskListOp/TaskOp aliases,
  and annotate fetch_task_lists accumulators.