refactor: share strip undefined helper

[chubes4]

Summary

• Move the repeated stripUndefined object helper into runtime-core's shared object utilities.
• Update CLI and runtime-playground call sites to import the shared helper instead of carrying local copies.

Testing

• npm run build
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Identified the duplicated helper cluster, made the small consolidation, and ran targeted verification. Chris remains responsible for review and merge.