_write_config_file() now reads each file listed under the credentials-files
top-level spec key and appends its contents to config.env after config vars.
Paths support ~ expansion. Missing files fail hard with sys.exit(1).
Also adds get_credentials_files() to Spec class following the same pattern
as get_image_registry_config().
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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