Reflow text in `adr show`

[corani]

Currently long lines get wrapped in adr show (correct), but if you add manual line-breaks in the source text (so you don't get ridiculously long lines in most text editors) the line-breaks are kept in the output text (not correct).

[corani]

Fixed in #108.

The root cause was that goldmark (used by glamour) renders soft line breaks inside list items as hard breaks rather than spaces. The fix preprocesses the ADR body before rendering: indented continuation lines (2-space indent, non-blank, not a new list item) are joined onto the preceding line. Blank lines are respected as paragraph separators and never joined across.