fix(github): make pullRequestComment commentTag templateable

[singeol]

docs/services/github.md already shows templated pullRequestComment.commentTag, but the implementation currently treats commentTag as a literal string.

This causes multiple Argo CD Applications associated with the same PR to overwrite one another's comments instead of upserting independently.

This change renders pullRequestComment.commentTag through the Go templating engine, adds test coverage, and updates the GitHub service docs.

This is a partial fix for #355 and addresses the templateable commentTag portion of #433.

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.16%. Comparing base (da04400) to head (7b82865).
⚠️ Report is 37 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/services/github.go	76.47%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
+ Coverage   55.41%   57.16%   +1.75%     
==========================================
  Files          46       49       +3     
  Lines        4125     3794     -331     
==========================================
- Hits         2286     2169     -117     
+ Misses       1511     1265     -246     
- Partials      328      360      +32     

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