Add public repository community files

[MikeChristensen]

Summary

• add adapted contribution and pull request guidance
• add Samples-specific bug and feature issue forms
• add code ownership for protected main changes
• add private security reporting guidance

Repository settings

Samples is now aligned with Core for squash-only merges, automatic branch deletion, read-only Actions tokens, Dependabot security updates, secret scanning with push protection, private vulnerability reporting, and main-branch rules. The required status check is adapted to Samples' actual check name: build.

Testing

• validated issue-form YAML structure against Core's templates
• confirmed the existing Samples build check is named build
• confirmed the branch is clean and the diff passes git diff --check