Normalize and align CD pipeline

[nanotaboada]

Summary

Improve the CD pipeline: extract a dedicated test gate before deployment, add multi-platform image support (amd64 + arm64), enable build provenance attestation, and tighten changelog generation.

Changes

Structure

• Extract a dedicated test job (currently tests run inside release)
• Add needs: test to the release job
• Add id-token: write and attestations: write permissions to release job

Test job steps

• Checkout repository
• Set up .NET
• Restore dependencies
• Build (Release configuration)
• Run tests

Release job

• Add linux/arm64 to platforms (currently linux/amd64 only)
• Set provenance: false → provenance: mode=max
• Add actions/attest-build-provenance@v4.1.0 step after image push with push-to-registry: true
• Add --no-merges flag to changelog git log command
• Add empty changelog guard: No new changes since $PREVIOUS_TAG when a previous tag exists, No changes (first release) otherwise
• Remove intermediate changelog.txt file write (unused outside the step)
• Drop Quick Start section from release body (belongs in README)

Acceptance Criteria

• CD triggers on v*.*.*-* tags and completes successfully
• test job fails fast if tests fail before any image is pushed
• Docker image published for both linux/amd64 and linux/arm64
• Build provenance attestation appears on the GitHub Release
• Changelog in release body excludes merge commits

[coderabbitai]

🔗 Related PRs

#176 - chore(ci): improve .NET CI workflow [merged]
#316 - ci: migrate Azure Pipeline from Windows/MSBuild to Linux/.NET CLI [merged]
#357 - feat: implement tag-based deployment with stadium release names (#356) [merged]
#445 - ci(cd): verify tag commit is reachable from master (#439) [merged]
#449 - docs: normalize README structure and add RELEASES.md [merged]

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