Add config validation for .vaultctl.yml

[f3rdy]

Description

load_config() in config.py does not validate the configuration file. Unknown keys are silently ignored, and typos (e.g. pasword: instead of password:) are not detected.

Proposed Solution

• Warn on unknown top-level keys
• Validate that vault_file and keys_file are strings
• Validate password sub-keys (env, file, cmd)
• Provide clear error message for invalid config

Files

• src/vaultctl/config.py (function load_config)

Priority

P2

[f3rdy]

Superseded by #34, which introduces CUE-based schema validation as the chosen approach. The original goal of validating .vaultctl.yml is preserved as a sub-goal there (built-in CUE schema for the config file), within the broader scope of also validating vault.yml ↔ vault-keys.yml cross-consistency.