GitHub: Add GitHubAttentionReport, reporting about important items

[amotl]

About

The idea is to generate a Markdown report including items that deserve your attention, bugs first.

• bug and important are significant labels that convey importance on GitHub.
• type-bug and type-important are used by Python.

Other criteria of "what's significant?", or "what needs attention?" can be added as we go.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 12 lines in your changes missing coverage. Please review.

Project coverage is 67.93%. Comparing base (783de95) to head (0ed4193).
Report is 93 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rapporto/github/core.py	87.50%	6 Missing ⚠️
src/rapporto/github/cli.py	88.88%	3 Missing ⚠️
src/rapporto/github/model.py	93.18%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   64.18%   67.93%   +3.74%     
==========================================
  Files           8        9       +1     
  Lines         497      580      +83     
==========================================
+ Hits          319      394      +75     
- Misses        178      186       +8     

Flag	Coverage Δ
unittests	67.93% <90.47%> (+3.74%)	⬆️

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