CCM-18514: Gitleaks Config Improvements

[jamesthompson26-nhs]

Summary

This PR updates the secret scanning wrapper to support WSL/local Linux shells as well as macOS.

The scan-secrets action previously used a BSD-style script invocation unconditionally when running in an interactive terminal. That worked on macOS, but failed in WSL because GNU script expects the command to be passed via -c. This change adds a small compatibility layer that detects which script variant is available and uses the correct invocation form.

Why

Local pre-commit runs in WSL were failing before gitleaks actually ran, even though the same check passed in GitHub Actions. CI was unaffected because the action already skips the interactive script path when running in a CI environment.

Validation

• Confirmed the updated shell script has valid syntax with bash -n
• Confirmed there are no editor-reported errors in the updated script
• Kept existing GitHub Actions behaviour unchanged while fixing the interactive local/WSL path

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.