Add community health documentation

[mcdigman]

Summary

• add a pyakima-specific security policy with private reporting instructions and scoped numerical-library boundaries
• add the liveness_primer code of conduct verbatim
• add contribution guidance for uv setup, strict quality gates, compatibility testing, documentation, benchmarks, releases, and AI-assisted contributions

Validation

• prek run --all-files
• skylos --strict --danger --secrets --ai-defects .
• byte-for-byte comparison of CODE_OF_CONDUCT.md with the current liveness_primer version
• verified private vulnerability reporting is enabled for this repository

AI assistance

Codex drafted and adapted the documentation; the final text was reviewed against the repository configuration, workflows, support matrix, and current liveness_primer source.