docs: document code coverage collection and CI upload

[github-actions]

What changed

The recent commit (#399) added native GitHub Code Quality coverage upload to the go-test CI job. This PR updates CONTRIBUTING.md to reflect that change.

Changes

• New "Code coverage" subsection under "Running tests" explains:
  • How to generate a local coverage profile (go test -coverprofile=...)
  • How to view it with go tool cover -func and go tool cover -html
  • How CI converts the profile to Cobertura format via boumenot/gocover-cobertura and uploads it to GitHub Code Quality
  • The prerequisite: Code Quality must be enabled in repository settings; until it is, upload errors are surfaced as warnings
• "Submitting changes" checklist updated to mention coverage upload alongside the existing lint/format/test checks

Why

Contributors checking CI output need to understand the new coverage step — particularly the warning-only behavior while Code Quality is not yet enabled — so they don't misread those warnings as failures.

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Warning

Protected Files — Push Permission Denied

This was originally intended as a pull request, but the patch modifies protected files. A human must create the pull request manually.

Protected files

• CONTRIBUTING.md

The push was rejected because GitHub Actions does not have workflows permission to push these changes, and is never allowed to make such changes, or other authorization being used does not have this permission.

Create the pull request manually

# Download the patch from the workflow run
gh run download 26478720979 -n agent -D /tmp/agent-26478720979

# Create a new branch
git checkout -b docs/ci-coverage-documentation-3db8a03bbffafec6 main

# Apply the patch (--3way handles cross-repo patches)
git am --3way /tmp/agent-26478720979/aw-docs-ci-coverage-documentation.patch

# Push the branch and create the pull request
git push origin docs/ci-coverage-documentation-3db8a03bbffafec6
gh pr create --title 'docs: document code coverage collection and CI upload' --base main --head docs/ci-coverage-documentation-3db8a03bbffafec6 --repo amalgamated-tools/goauth

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