fix: preserve phpunit diagnostic artifact

[chubes4]

Summary

• Persist PHPUnit .pg-test-result.txt diagnostics from the Playground VFS back to the matching readwrite host mount before classifying command failures.
• Cover the regression with a focused smoke test that asserts a failing PHPUnit run leaves the diagnostic artifact on the mounted plugin directory.

Closes #97

Test evidence

• npm run build
• npm run phpunit-diagnostic-artifact-smoke
• npm run core-phpunit-command-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Root-cause investigation, implementation, focused smoke coverage, and verification. Chris remains responsible for review and merge.