data ingest: accept a file or a folder + instruction clarity + path bugs (RFC-0002 phase 3)

[LukasWodka]

Phase 3 of RFC-0002. Flexible input + clearer instructions + path bugs.

• Accept a bare .csv (tabular/time) as well as a directory; error clearly which is expected per family.
• Per-family layout help in --help (today only image_classification is documented for all tasks); rename the arg to <dataset>.
• Fix: ~user mis-expansion in expandHome; run the path-existence check before table/category validation so a typo'd path surfaces first.

Design-of-record: RFC-0002 (#174). Part of the data-ingest redesign epic. (§4, instruction-clarity findings)

[LukasWodka]

Status: merged to develop via cli#202 (2026-07-10). Promotion-pending — closes at prod (RFC-0002 epic #1008).