Tab on a list item only prepended two spaces and kept the marker, so
`1. a` / Shift+Enter / Tab produced `  2. ` instead of a nested `1.`.
Two spaces under `1. ` is also not nesting in CommonMark (the child must
reach the parent's content column), so the agent received a flat list.

- Tab nests the item at the content column of the item above and numbers
  a nested ordered list from 1, or continues an existing sublist.
- Shift+Enter on an empty nested item moves it up one level and continues
  the parent's numbering (the rich-text composer already did this; the
  plain composer left the whole list).

Fixes #17124