Add CONTRIBUTING and Contributor Covenant 2.1 Code of Conduct

[wnstfy]

Summary

• Adds CONTRIBUTING.md describing how to propose changes, run pre-commit hooks, and the bar each stack must meet.
• Adds CODE_OF_CONDUCT.md — the official Contributor Covenant 2.1 text, fetched verbatim from contributor-covenant.org. Reporting contact (contact@webnestify.cloud) matches SECURITY.md.
• Completes the OpenSSF Best Practices community-files set alongside the existing SECURITY.md and LICENSE.

Test plan

• scan (PR CVE gate) passes — no code or image changes, expected clean.
• actionlint / pr-misconfig-scan workflows trigger but find nothing (no workflow or compose changes).
• Files render correctly on GitHub (CoC contact link, CONTRIBUTING headings).

[github-actions]

Trivy misconfiguration findings (HIGH + CRITICAL)

Findings below. Fix the issue or add an ignore entry to .trivyignore with a justification.

Report Summary

┌────────┬──────┬───────────────────┐
│ Target │ Type │ Misconfigurations │
├────────┼──────┼───────────────────┤
│   -    │  -   │         -         │
└────────┴──────┴───────────────────┘
Legend:
- '-': Not scanned
- '0': Clean (no security findings detected)