Add recommended labels documentation

[Hartwich]

Goal

Document recommended GitHub labels for Open Party Lab and game repos.

Codex Auftrag

Add a short docs file or CONTRIBUTING section.

Focus

• agent-ready
• good first issue
• playtest-needed
• area:host/controller/server/i18n/assets/gameplay/balancing/docs
• needs-design
• needs-clarification
• size:xs/s/m/l

Acceptance Criteria

• Add a short docs file or CONTRIBUTING section.
• Explain when each label should be used.
• Do not create issues.

Safety

Do not add copyrighted, unclear, or unreviewed third-party assets or content.