feat(notify): add custom header for github issue

[franzramadhan]

Summary

Add custom header functionality to the github issue notification to override the default format # Compliance Report — {org}.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.35%. Comparing base (cc28f98) to head (8360e2a).

Files with missing lines	Patch %	Lines
cmd/git-cascade/cmd/scan.go	20.00%	4 Missing ⚠️
internal/notify/issues.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   72.41%   72.35%   -0.06%     
==========================================
  Files          34       34              
  Lines        2686     2695       +9     
==========================================
+ Hits         1945     1950       +5     
- Misses        655      659       +4     
  Partials       86       86              

Flag	Coverage Δ
unittests	72.35% <64.28%> (-0.06%)	⬇️

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