chore: add dependency security automation

[Mustapha3415]

Summary

• Configure weekly npm dependency updates with separate patch and minor groups.
• Add CodeQL analysis and pnpm audit checks for pull requests and scheduled scans.
• Enable patch-only Dependabot auto-merge after required checks pass.
• Document the repository settings administrators must enable for the dependency graph, alerts, security updates, auto-merge, and branch protection.

Closes #73

[github-advanced-security]

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