Validate Base-generated environment reports

[codeforester]

Summary

• Demonstrate basectl devcontainer and basectl devenv-report as read-only JSON compatibility reports.
• Add macOS and Ubuntu CI steps that run both commands against the pinned Base checkout and assert stable JSON fields.
• Update README, AI context, the tooling matrix, contract registry, changelog, and validation guards.

Fixes #182.

Validation

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