feat: add neovim lua plugin scaffold

[dkarter]

Motivation

Add the Neovim-native Lua plugin shell before porting behavior incrementally.

Summary of Changes

• Adds plugin/bullets.lua and require("bullets").setup({}).
• Adds Lua config defaults, command registration, <CR> mappings, and buffer-local mapping setup.
• Updates tests and tooling to load plugin/bullets.lua.
• Migrates test helper defaults to setup({}) instead of vim.g globals.
• Enables only the plugin-load smoke test.

Testing

• mise run fmt
• mise run test

Dependencies/Special Considerations

Linear: RMS-67

Stack root after RMS-77 merged.

Stack

1. feat: add neovim lua plugin scaffold #6 👈 current
2. feat: continue standard bullets #4
3. feat: support ordered bullet continuations #7

Stack

1. feat: add neovim lua plugin scaffold #6 👈 current
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
9. chore: add release please #14
10. docs: add lazy.nvim usage #15

[dkarter]

I don't think we need this for neovim plugins - right?