Document future Go Cobra CLI boundary

[codeforester]

Summary

• Add a Future Go/Cobra CLI Boundary section to the representative environment design.
• Decide that a non-trivial Go/Cobra CLI stays out of the current baseline demo.
• Name base-demo-go or an optional advanced slice as the future split path and guard the decision in validation.

Closes #13

Validation

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