GitHub: Refactor and spread GitHub modules along the feature axis

[amotl]

About

Refactor and spread GitHub modules along the feature axis, away from including all into a single core.py file.

Naming things

• Actions: Report about CI failures on GHA.
• Activity: Report about user activity in PPP format.
• Attention: Report about bugs and similar important items.

[codecov]

Codecov Report

Attention: Patch coverage is 89.53069% with 29 lines in your changes missing coverage. Please review.

Project coverage is 71.25%. Comparing base (a925c04) to head (5ea537e).
Report is 75 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rapporto/github/actions.py	82.24%	19 Missing ⚠️
src/rapporto/github/activity.py	94.05%	6 Missing ⚠️
src/rapporto/github/attention.py	93.75%	3 Missing ⚠️
src/rapporto/github/util.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   70.41%   71.25%   +0.84%     
==========================================
  Files           9       11       +2     
  Lines         649      668      +19     
==========================================
+ Hits          457      476      +19     
  Misses        192      192              

Flag	Coverage Δ
unittests	71.25% <89.53%> (+0.84%)	⬆️

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