This change makes Terraform plugin cache cleanup user-configurable with minimal surface area:

• Replace hard-coded 30d retention with a configurable variable
• Read environment in provisioner Serve():
  • CODER_PROVISIONER_CACHE_CLEANUP (default true)
  • CODER_PROVISIONER_CACHE_RETENTION (e.g. 720h)
• Guard the cleanup call in Plan() with the toggle
• Update docs (best practices) to describe the new env vars

Behavior:

• Defaults remain unchanged (cleanup enabled; ~30d retention)
• Works for both built-in and external provisioners (env read in Serve)

Notes:

• Tests in provisioner/terraform require a Terraform binary; build succeeds locally. Happy to add CI setup or adjust tests if needed.