chore: harden repository trust and security

[AmirMotefaker]

Closes #7

Summary

Hardens repository trust, community contribution quality, GitHub Actions supply-chain security, and vulnerability reporting.

Delivered

• Security policy and private vulnerability reporting.
• Contributing guide and Code of Conduct.
• Issue forms, PR template, and CODEOWNERS.
• Dependabot version updates.
• Vulnerability alerts and Dependabot security updates.
• Read-only default GITHUB_TOKEN.
• GitHub Actions cannot approve PRs.
• Every action reference pinned to a full 40-character commit SHA.
• Repository Actions policy requires full-SHA pinning.
• CodeQL where technically appropriate.
• Existing license state preserved exactly.

Validation

• Official action tag -> commit mappings verified before branch generation.
• All workflow uses: references statically verified as 40-character SHAs.
• git diff --cached --check passed.
• License snapshot verified unchanged.
• Repository security settings verified after mutation.
• Conflicting CodeQL default setup is disabled when advanced workflow setup is used.
• GitHub Actions PR checks are mandatory before merge.

Evidence

• evidence/phase6-trust-security-hardening.md
• Evidence SHA-256: 3C0F73BD028D93F5FB914C5A04F545964135A737B1069D69CD9D080DB99D74DD
• Head SHA: 9c42b46de01304fa3b14282558843bea9828fbfc
• Community health before: 28%

[github-advanced-security]

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For more information about GitHub Code Scanning, check out the documentation.