Use mounted workspace backend in sandbox runs

[chubes4]

Summary

• Force Data Machine Code sandbox runs to use the mounted /workspace backend instead of the remote GitHub backend.
• Keep the existing sandbox workspace root and adoption flow.
• Preserve WP Codebox recipe-run diagnostics when @wp-playground/cli calls process.exit() during runtime boot.
• Cover the emitted backend filter in the agent sandbox code smoke.

Tests

• npm run build
• npm run agent-sandbox-code-smoke
• Homeboy Lab targeted probe verified workspace_read and workspace_write with a non-empty patch artifact.

Depends on Extra-Chill/data-machine-code#459
Also depends on Extra-Chill/data-machine#2321 for opt-in allow-policy tools.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Traced WP Codebox sandbox workspace failures through Data Machine/Data Machine Code policy and backend selection, drafted the sandbox backend and diagnostics fixes, rebuilt the CLI, and ran focused Homeboy Lab verification.