Materialize staged mount directories

[chubes4]

Summary

• Materializes directory mount targets and empty/intermediate directories into the Playground VFS.
• Keeps skipped-directory policy for large/cache trees while preserving readable mounted directory structure.
• Extends mount materialization coverage for nested and empty directories.

Testing

• npx tsx tests/mount-materialization.test.ts
• npm run test:recipe-runtime-setup-staged-materialization
• npx tsx tests/staged-input-materialization.test.ts
• npm run build -- --pretty false

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Debugging the staged mount directory materialization gap, implementing the follow-up fix, and verifying focused coverage.