Add GitHub Actions job summary support for issues summary

[Copilot]

This PR implements GitHub Actions job summary support for the issues summary feature, enabling builds running on GitHub Actions to display a formatted summary of code issues directly in the job summary view.

What's Changed

Previously, the CreateSummaryIssuesReport method in GitHubActionsBuildServer was not implemented and simply contained a comment stating "Summary issues report is not supported for GitHub Actions." This meant that when running Cake Issues Recipe on GitHub Actions with summary reports enabled, users would not see any summary output.

This change implements the missing functionality by:

1. Creating markdown templates (GitHubActionsSummary.cshtml) for both Cake Script and Frosting versions that format issues into a clean, readable table
2. Implementing the CreateSummaryIssuesReport method to generate the summary and append it to the $GITHUB_STEP_SUMMARY environment variable file
3. Following the same pattern as the existing Azure DevOps implementation for consistency

How It Works

When issues summary is enabled and the build runs on GitHub Actions:

• A markdown summary is generated using the new template
• The summary displays issues in a formatted table with columns for severity, project, directory, file, line, rule, and message
• If no issues are found, a positive "✅ No issues found in the code" message is shown
• The content is automatically appended to GitHub's job summary via the $GITHUB_STEP_SUMMARY environment variable
• Appropriate logging messages inform users of the operation status

Example Output

When issues are found:

# Issues Summary

There are **3** issues in the code.

| Severity | Project | Directory | File | Line | Rule | Message |
|----------|---------|-----------|------|------|------|---------|
| Warning  | MyApp   | src/      | Program.cs | 42 | CS0168 | Variable is declared but never used |

When no issues are found:

# Issues Summary

✅ **No issues found in the code.**

Both Cake Script and Frosting versions are kept in sync and maintain identical functionality.

Fixes #610.

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