Add open-source connected service foundation

[SebastianSteele]

Summary

• add tested optional workspace API and outbound mail relay clients
• define the PostgreSQL, OpenAPI, and self-hosting trust boundary
• add public contribution, conduct, security, and issue-reporting files
• replace the repository explanation with reproducible production screenshots

Checks

• Node 20.19.0 typecheck
• 21 tests
• production build
• npm audit at high severity: 0 vulnerabilities
• Chrome 150 desktop screenshot review

Current repository blocker

GitHub Actions is currently blocked at the account level before any job starts. The same workflow passes locally and can be rerun after the account lock is cleared.