sticky comment

[CodeYogiCo]

❤ THANKS FOR YOUR PULL REQUEST

Proposed change

Describe your change and communicate why we should accept the pull request. Links to issues are helpful too, if applicable.

Types of changes

What types of changes is this pull request introducing to this plugin? Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

You can fill this out after creating your PR. Put an x in the boxes that apply

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[CodeYogiCo]

What this PR changes

This bundles several improvements to how the plugin obtains the diff and reports coverage.

Diff sourcing

• Diff logic moved into the plugin. With PARAMETER_DIFF_SOURCE=github the plugin fetches the PR's unified diff directly from the GitHub API (GET /repos/{owner}/{repo}/pulls/{n} with the vnd.github.v3.diff media type) instead of requiring a piped git diff. stdin remains the default, so existing usage is unchanged.
• Removed the Vela-specific scripts/start.sh entrypoint. The Docker image now runs the binary directly (ENTRYPOINT ["/plugin"]). The stdin path still works by piping a git diff into the container.

Reporting

• Sticky PR comment. Instead of posting a new comment on every push, the plugin now looks for its previous comment (via a hidden marker) and updates it in place — one always-current coverage comment per PR.
• Job summary output. A new reporter writes the same Markdown report to $GITHUB_STEP_SUMMARY, so coverage shows on the Actions run page — visible even on fork PRs where the read-only token can't post a comment.

CI

• The coverage job now also runs on fork PRs (the read-only-token comment failure is tolerated via continue-on-error on forks), so contributors from forks still get coverage in the job summary.

Internals / tests

• Extracted a shared buildMarkdownReport used by both the PR comment and the job summary.
• Added Unmarshal to the JSON client (to read the existing-comments list for the sticky behavior).
• Updated tests (incl. a sticky-update case and a step-summary reporter test), the mock GitHub API, and README/AGENTS docs. go test ./..., gofmt, and golangci-lint are all clean.