resolveConfig cache ignores options on subsequent calls

[oritwoen]

resolveConfig() caches the first result and returns it for all subsequent calls, even when different options (cwd, envName, overrides, etc.) are passed. This means only the first call's options take effect.

Found during review of #12.

Options:

• Skip cache when any option is explicitly passed
• Invalidate cache when options differ from the cached call
• Document that resolveConfig is single-shot by design