fix: escape sandbox permission helper reference

[chubes4]

Summary

• Escape the generated PHP namespace separator for the Data Machine PermissionHelper class reference.
• Add smoke coverage so the sandbox generated code keeps the namespaced class reference and does not collapse it to DataMachineAbilitiesPermissionHelper.

Verification

• npm run agent-sandbox-code-smoke
• npm run build
• git diff --check

Evidence

• Follow-up to failed site-generation proof run: https://github.com/chubes4/wp-site-generator/actions/runs/26976447576
• Failure showed Class \"DataMachineAbilitiesPermissionHelper\" not found from the WP Codebox-generated PHP bridge.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Diagnosed the proof-run artifact failure, drafted the minimal escaping fix, added smoke assertions, and ran local verification. Chris remains responsible for review and merge.