feat: support nested checkboxes

[dkarter]

Motivation

Implement Linear RMS-73 by adding nested checkbox continuation, toggling, and recomputation support.

Summary of Changes

• Continue checkbox bullets with the configured unchecked marker.
• Toggle current checkboxes and propagate parent/child state through nested checkbox trees.
• Support configured partial and UTF checkbox marker strings via setup({ checkbox_markers = ... }).
• Enable the migrated checkbox specs without deleting pending coverage.

Testing

• mise run fmt:check
• mise run test

Dependencies/Special Considerations

Linear: RMS-73

Stacked on PR #12 / dorian/rms-71-implement-bullet-list-renumbering.

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
6. feat: support outline promotion #11
7. feat: renumber bullet lists #12
8. feat: support nested checkboxes #13 👈 current
9. chore: add release please #14
10. docs: add lazy.nvim usage #15