feat: handle empty bullets and spacing

[dkarter]

Motivation

Implements RMS-68 newline behavior for empty generated bullets, configured blank spacing, ordered-bullet padding, and colon-triggered child indentation.

Linear: RMS-68

Summary of Changes

• Handle delete_last_bullet_if_empty values for deleting, retaining, or promoting empty generated bullets.
• Insert configured line_spacing blank lines and respect pad_right during newline continuation.
• Add auto_indent_after_colon behavior for : and fullwidth ：.
• Enable focused newline behavior specs while leaving unrelated pending specs pending.

Testing

• mise run fmt:check
• mise run test

Dependencies/Special Considerations

Stacked on PR #9 (dorian/rms-70-support-document-specific-and-wrapped-bullets).

Stack

1. feat: add neovim lua plugin scaffold #6
2. feat: continue standard bullets #4
3. feat: support ordered bullet continuations #7
4. feat: support wrapped and document bullets #9
5. feat: handle empty bullets and spacing #10 👈 current
6. feat: support outline promotion #11
7. feat: renumber bullet lists #12
8. feat: support nested checkboxes #13
9. chore: add release please #14
10. docs: add lazy.nvim usage #15