feat(shell): route native caches externally

[Saltiola7]

Summary

• route Playwright, uv, pre-commit, npm, and Pulumi through documented native path controls
• apply only to machine_type = mac-mini when the external-state sentinel exists
• preserve inherited custom values and internal defaults for every other machine

Validation

• 7 focused terminal-environment tests pass
• shell present/absent-sentinel behavior and native tool smoke checks passed
• DBSCTR gates passed; helper Final Push was unavailable only because the explicitly approved push destination changed from a local path to GitHub