Add tests for:
- @Done insertion at various line positions
- Future task filtering with show_future flag
- Task sorting by moment ascending
- Error message formatting for all new error variants
- Trailing newline preservation
Implement smoother todo editing with the following features:
- Display numbered tasks [1], [2], etc. in `streamd todo`
- Add `streamd todo N edit` to open editor at task line
- Add `streamd todo N done` to insert @Done after @Task
- Add `--show-future` flag to include future tasks (hidden by default)
