docs: add cross-project idea briefs

[rodrigobnogueira]

Summary

• add idea briefs for the production reference app, transactional outbox package, and future app generator
• keep outbox framed as a separate project that can integrate with Drizzle and tRPC, not as Drizzle package scope

Validation

• reviewed idea headers for project type and ownership

Security

• documentation-only change
• no secrets, dependencies, scripts, or runtime behavior changed