Covers the last missing piece of Jalon 0 in ROADMAP.md:
'CONTRIBUTING.md (build, tests, conventions, DDD sessions)'
Sections:
- prerequisites (.NET 10, PostgreSQL, Node for Avalonia Browser,
Android SDK for PostIt.Android)
- first build + how to start Yavsc.Org in Development
- how to run the test suite
- code conventions (delegated to .editorconfig + a few extras)
- branches and commit messages (trunk-based, scoped imperatives)
- link to architecture/decoupage-organisation.md for the
per-project layout
- DDD sessions (link to doc/ddd-exploration-*.md + ROADMAP.md)
- security reminders (no secrets in git, user-secrets / env vars)
- pointer to GitHub issues + new DDD sessions for design Qs
