Add CONTRIBUTING.md

[zhiganov]

Issue templates exist (bug report, feature request) but there's no contributor guide.

What's needed:

• How to set up local dev (point to Docker setup, Add Docker Compose for local development #58)
• Branch naming convention (feature/, fix/)
• PR process (branch off master, describe changes)
• Code style (TypeScript strict, Prettier config, RSC preference)
• Where to find architecture docs (docs/architecture.md, CLAUDE.md)
• Note about shared Neon database with Pro — OSS contributors should use local Postgres via Docker, not the production DB