Add community health files: CONTRIBUTING.md, CODE_OF_CONDUCT.md, SUPPORT.md

[zeevdr]

Description

Add standard community health files to make the project welcoming and professional for external contributors.

Acceptance criteria

• CONTRIBUTING.md — how to contribute (open issue before PR, code style, testing expectations, PR process)
• CODE_OF_CONDUCT.md — Contributor Covenant v2.1
• SUPPORT.md — where to go for help, which repo to file issues in (core vs SDK vs UI)
• Link to these from the main README

Notes

• These should live in the main decree repo (or org-level .github repo after org migration)
• SUPPORT.md is especially important for a multi-repo project — users will file issues in the wrong repo
• Keep CONTRIBUTING.md concise — nobody reads a 10-page guide