feat: support outline promotion

[dkarter]

Motivation

Implement RMS-72 outline promotion and demotion for bullets.nvim, stacked on PR #10.

Linear: RMS-72

Summary of Changes

• Adds parser-based BulletPromote, BulletDemote, and visual range variants.
• Uses setup({ outline_levels = ... }) to control level transitions.
• Removes the bullet when promoting at top level and preserves the final standard bullet behavior when demoting beyond configured levels.
• Wires visual command ranges and keeps renumber_on_change as the integration hook.
• Enables focused nested bullet specs for promotion/demotion coverage.

Testing

• mise run fmt:check
• mise run test

Dependencies/Special Considerations

Stacked on PR #10 / dorian/rms-68-implement-empty-bullet-spacing-and-colon-behavior.

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 👈 current
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