…kdown

When a Changelog Entry section contains plain text (no bullets), multi-line
content is now joined with spaces to produce valid markdown output.

Previously, newlines were preserved which resulted in broken output like:
- This is a multi-line
changelog entry that
spans several lines. by @user in [#1](url)

Now produces:
- This is a multi-line changelog entry that spans several lines. by @user in [#1](url)

Also adds regression test for this case.

Previously, nested bullets like:
  - Add feature
      - Sub-item 1
      - Sub-item 2

Would render as:
  - Add feature by @user in [#1]
    Sub-item 1
    Sub-item 2

Now correctly renders as:
  - Add feature by @user in [#1]
    - Sub-item 1
    - Sub-item 2

Nested items do NOT get author/PR attribution - only the top-level entry does.