chore(ci): fix S3 artifacts bucket references in install step and secure terraform tfvars

- Use DEV_S3_ARTIFACTS_BUCKET and PROD_S3_ARTIFACTS_BUCKET in 06_install
  instead of generic S3_ARTIFACTS_BUCKET to prevent cross-env reads
- Add terraform/environments/*.tfvars to .gitignore to prevent secret leaks
- Update prod backend state bucket name to proyectosacc-specific bucket
- Add CI/CD credential policy documentation

terraform/backend.prod.hcl

@@ -1,4 +1,4 @@
-bucket         = "ccsoft-terraform-state-739086995772"
+bucket         = "ccsoft-proyectosacc-terraform-state-prod"
 key            = "proyectosacc/terraform.tfstate"
 region         = "mx-central-1"
 encrypt        = true