Document Ubuntu setup CI boundary

[codeforester]

Summary

• update README and CONTRIBUTING to reflect the current macOS/Ubuntu boundary after Base v1.6.1
• document that Ubuntu/Debian CI now validates Base runtime setup, Base --profile dev, and read-only project health
• keep the full base-demo project setup, activation, build, test, and demo loop documented as macOS-only in this repository
• update the contract registry and validation guards for the new boundary
• add the same platform-boundary note to AI context overview

Root Cause

After PR #166, the Ubuntu job no longer performs a manual venv bootstrap. It validates the supported Base Ubuntu setup path and dev-profile package mapping. The docs still described Ubuntu as only read-only CI, which understated what is now proven while still needing to preserve the macOS-only boundary for the full base-demo project loop.

Closes #161.

Validation

• ./tests/validate.sh
• git diff --check