Add Community Guidelines Files (CONTRIBUTING, CODE_OF_CONDUCT, SECURITY)

[Lyokone]

Add essential community files to enable external contributions and establish project standards.

Priority: P0 (Critical)
Effort: 2 days
Labels: documentation, community, good-first-issue

Acceptance Criteria

• Create CONTRIBUTING.md with:
  • Development setup instructions
  • Code style guidelines (refer to existing analysis_options.yaml)
  • Commit message format (conventional commits)
  • Pull request process
  • Testing requirements
  • Branch naming conventions
• Create CODE_OF_CONDUCT.md:
  • Use Contributor Covenant v2.1
  • Add enforcement contact information
• Create SECURITY.md with:
  • Supported versions table
  • Vulnerability reporting process (email, security advisory)
  • Response timeline expectations
  • Security disclosure process
  • Known security considerations (service account handling)
• Link all three files in main README.md

Technical Notes

• Follow GitHub's community standards recommendations
• Reference Invertase's other projects for consistency
• Include Firebase-specific security considerations

Definition of Done

• All three files created and reviewed
• Links added to README
• Community health score improves in GitHub insights