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

.gitignore

@@ -19,6 +19,8 @@ terraform/override.tf
 terraform/override.tf.json
 terraform/*_override.tf
 terraform/*_override.tf.json
+terraform/environments/*.tfvars
+!terraform/environments/*.example.tfvars
 
 # Claves SSH y certificados
 *.pem