fix(remediation): use conventional-commit PR titles and sync existing PRs

[franzramadhan]

Summary

• Remediator PR titles now follow Conventional Commits (fix(<rule-id>): <description>) instead of a git-cascade: prefix.
• CreateOrUpdatePullRequest now edits the title/body of an already-open PR on the same head branch when they've drifted from the current fix, instead of leaving stale titles/bodies untouched on rerun.

Test plan

• go build ./...
• go vet ./...
• go test ./...
• Added TestCreateOrUpdatePullRequest_UpdatesTitleWhenChanged and renamed the no-op case to TestCreateOrUpdatePullRequest_ReturnsExistingWhenOpenAndUnchanged

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.06%. Comparing base (3c43703) to head (75d1ca2).

Files with missing lines	Patch %	Lines
internal/github/write.go	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   78.06%   78.06%   -0.01%     
==========================================
  Files          41       41              
  Lines        3150     3159       +9     
==========================================
+ Hits         2459     2466       +7     
- Misses        549      550       +1     
- Partials      142      143       +1     

Flag	Coverage Δ
unittests	78.06% <85.71%> (-0.01%)	⬇️

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