chore(badges): add OpenSSF Best Practices badge to README

[bigwellaadmin-dev]

Project 12980 on bestpractices.dev passed the self-assessment on first registration. Adding the badge alongside the existing Scorecard and licence/citation badges so the security posture is visible from the landing page.

Pull Request

What this change does

Briefly describe the change in one or two sentences.

Type of change

• Tool fix (bug, accessibility, performance)
• Skill improvement
• Documentation or guide edit
• Rubric or assessment update
• New translation or adaptation

Important: framework content is canonical

The framework text (the six steps, the F check questions, the Y diagnostic, the in-one-sentence lines) is the published intellectual work of Sandra Robinson and is treated as canonical. Changes to the framework content itself need to be discussed in an issue first before a PR is opened.

Implementation changes (the tool, the skill instructions, the website copy, the build specs) are open to PR without prior discussion.

Checklist

• I have read the Code of Conduct
• I have read CONTRIBUTING.md
• The framework text is unchanged (or has been discussed in an issue first)
• If I changed the tool, I tested it in Chrome / Firefox / Safari
• If I changed the skill, I tested it in a Claude project