Handle missing git user config gracefully

[fingolfin]

Check for missing git user.name and user.email before creating the
initial commit, prompt the user to retry after configuring git,
and allow aborting git setup without discarding the generated
package directory.

Add regression coverage for the success, abort, and retry paths,
and document the behavior in the README and package manual.

Fixes #55

Co-authored-by: Codex codex@openai.com

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 55.58%. Comparing base (027ece7) to head (f56bd8a).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/generate.g	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   54.19%   55.58%   +1.39%     
==========================================
  Files           9        9              
  Lines         692      716      +24     
==========================================
+ Hits          375      398      +23     
- Misses        317      318       +1     

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