test: add external dogfood harness for configure-nodejs

[huntharo]

Summary

• I added a dedicated dogfood harness repository for pwrdrvr/configure-nodejs
• I copied the npm, pnpm, and Yarn fixtures into this repo and added unit tests that run against a checked out configure-nodejs worktree
• I added a workflow that checks out pwrdrvr/configure-nodejs at a configurable ref, runs the helper-script unit tests, and exercises fixture cache lookup plus restore behavior through the action entrypoint

Verification

• I verified the workflow YAML parses locally with Ruby's YAML loader
• CI on this branch will validate the harness end to end against pwrdrvr/configure-nodejs