Add base-platform-tools to workspace example

[codeforester]

Summary

• Add optional base-platform-tools to workspace.yaml.example.
• Explain the optional companion in the workspace discovery walkthrough when Base reports it.
• Update README and repository validation guards for the workspace example contract.

Fixes #151

Validation

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