Publish rich GitHub Step Summary with change matrix and runner info

[CasperKristiansson]

Summary

Enhance the action output by writing a GitHub Step Summary that includes the change matrix, runner availability status, and any security gate reasoning, while keeping the existing JSON outputs intact.

Why

Maintainers reviewing runs in Actions logs benefit from a concise summary without digging into raw logs. GitHub Step Summaries provide a supported way to surface human-readable context.

What needs to happen

• Detect when the action runs within GitHub Actions and a GITHUB_STEP_SUMMARY file is available.
• Render a markdown summary covering: files changed, proposed version, runner availability (from existing data or the new gate), and reasons for skips/security gating.
• Keep the existing structured outputs the same for integrations.
• Add tests around the summary renderer and include snapshots to guard formatting.
• Document the addition so users know what to expect in the Actions UI.

References

• Roadmap hardening item: "Better run summary".
• GitHub Actions docs for Step Summary support.

Acceptance criteria

• Successful runs write a markdown summary file when supported.
• Summary gracefully skips when not running in Actions or when file path is unavailable.
• Tests cover at least one change scenario and one skip scenario.