Configuration & credentials handling

[chmmou]

Profile-aware config loader for KAS credentials.

• Config file (TOML) under XDG config path, multi-profile.
• Env var fallback: KAS_LOGIN, KAS_AUTHDATA, KAS_AUTHTYPE.
• Selection precedence: flag > env > config-profile > default profile.
• Make sure secrets never appear in --help or in the default log output.
• Unit tests for precedence and missing-credential errors.