Document the GitHub-native contribution workflow

[Happypig375]

Summary

• replace obsolete status/resolution-label guidance with the repository's GitHub-native workflow
• direct open-ended questions and ideas to Discussions while keeping issues actionable
• document draft PRs, milestones, sub-issues, dependencies, and issue state reasons
• add a concise pull request template for scope, issue linkage, validation, compatibility, maintenance cost, and review findings

Validation

• git diff --check
• two independent documentation/template reviews: approved