Add issue planning board / roadmap document

[Hartwich]

Goal

Add a lightweight roadmap document for Open Party Lab.

Codex Auftrag

Create a high-level roadmap that is easy to update.

Focus

• platform improvements
• game improvements
• AI-agent-ready tasks
• playtesting tasks
• asset/legal review tasks

Acceptance Criteria

• Roadmap is high-level and easy to update.
• Does not promise fixed release dates.
• Separates platform and game repo tasks.

Safety

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