Add security policy and contribution safety guardrails

[nimrodkra]

Summary

• Add a repository SECURITY.md policy tailored to content and link-safety risks.
• Add Dependabot updates for GitHub Actions to keep workflow dependencies current.
• Tighten contributor intake by requiring canonical URL hygiene and affiliation disclosure in templates/guidelines.

Test plan

• Validate changed markdown/yaml files for lint diagnostics in editor.
• Review diff to confirm only governance/safety files are included.
• Verify branch protection and private vulnerability reporting settings in GitHub UI.

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