ci: skip redundant image publishes

[haasonsaas]

Summary

• Skip Publish image for docs/Markdown-only main pushes.
• Cancel superseded main publish runs so only the latest image publish continues.

Evidence

Recent GitHub Actions timing showed DiffScope Publish image at 21 sampled main-push runs and ~177 workflow-minutes, with Docker publish jobs commonly taking 8-12 minutes. Docs-only merges do not need to rebuild the image used by PR review automation.

Validation

• git diff --check
• actionlint .github/workflows/publish-image.yml

[cursor]

PR Summary

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