feat: reuse OpenAI assistant

[mtth]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.39%. Comparing base (8078161) to head (8bc00b1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/git_draft/bots/__init__.py	60.00%	1 Missing and 1 partial ⚠️
src/git_draft/bots/common.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   94.15%   94.39%   +0.24%     
==========================================
  Files           8        8              
  Lines         359      357       -2     
  Branches       27       27              
==========================================
- Hits          338      337       -1     
+ Misses         15       13       -2     
- Partials        6        7       +1     

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