feat: add completions-based OpenAI bot

[mtth]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.24%. Comparing base (3f83752) to head (537600f).

Files with missing lines	Patch %	Lines
src/git_draft/bots/__init__.py	57.14%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
- Coverage   94.59%   93.24%   -1.35%     
==========================================
  Files           8        8              
  Lines         370      385      +15     
  Branches       28       30       +2     
==========================================
+ Hits          350      359       +9     
- Misses         13       18       +5     
- Partials        7        8       +1     

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