Add security threat model documentation

[ChrisJBurns]

Summary

Adds a comprehensive security threat model for the Releaseo GitHub Action in SECURITY.md.

Contents

• Trust boundaries: Diagrams showing where untrusted data enters the system
• Data flow: How inputs flow through action.yml to the Go binary
• Asset inventory: Critical assets (tokens, files) with sensitivity ratings
• Threat actors: Malicious workflow authors, PR authors, compromised dependencies, insiders
• Threat enumeration: 8 threats analyzed using STRIDE methodology
  • T1: Shell injection via inputs ✅ Mitigated
  • T2: Command injection via helm-docs ✅ Mitigated
  • T3: Path traversal ✅ Mitigated
  • T4: YAML injection ✅ Mitigated
  • T5: Token exposure ⚠️ Partially mitigated
  • T6: DoS via large files ⚠️ Low risk
  • T7: Supply chain attacks ⚠️ Recommendations provided
  • T8: Git branch manipulation ⚠️ Low risk
• Security controls: Summary of implemented and recommended controls
• Token permissions: Minimum required GitHub token permissions
• User checklist: Security best practices for action users

Test plan

• Document renders correctly in GitHub markdown
• Review by security team

🤖 Generated with Claude Code