chore: externalize configure-nodejs dogfood coverage

[huntharo]

Summary

• I switched the action to explicit cache restore/save steps so it only saves when a node_modules directory actually exists
• I clarified that package-manager is an optional override and the default behavior follows the detected lockfile metadata
• I moved the dogfood CI workflow, fixtures, and helper-script unit tests out of this repo into pwrdrvr/configure-nodejs-test so the published action repo stays leaner

Verification

• I verified the action metadata YAML parses locally with Ruby's YAML loader
• Dogfood coverage now lives in pwrdrvr/configure-nodejs-test and will exercise the action from a separate harness repository