chore: harden release CI, add SECURITY.md, LICENSE and CODEOWNERS

[prajwol-uxcam]

Summary

• Harden the release workflow against supply-chain attacks: SHA-pin all third-party actions, set least-privilege permissions, add step-security/harden-runner in audit mode, set persist-credentials: false on actions/checkout, create tags via gh CLI.
• Replace deprecated slack-github-action with v3.
• Add a separate zizmor audit workflow that scans .github/workflows/ on every PR and pushes findings to GitHub Code Scanning.
• Add CODEOWNERS so pubspec.yaml requires maintainer review, and any change under .github/ requires either @uxcam/maintainers or @uxcam/security-team.
• Update LICENSE for consistency

[github-advanced-security]

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