chore: add public contribution governance

[barbatdev]

Fixes #29

Summary

• add CONTRIBUTING.md with issue-first, PR-only, maintainer-review guidance
• add CODEOWNERS to route repository-owner review
• add pull request and issue templates for focused public contributions

Validation

• scripts/oss-audit.sh
• bash -n scripts/oss-audit.sh
• bash -n install.sh

Notes

• No branch protection or ruleset was changed automatically because CODEOWNERS/self-review behavior is ambiguous when the PR author and sole code owner may be the same account.