lint: add --config CLI argument for custom config path

[reidbaker]

Adds the -c / --config CLI argument to dart_skills_lint to support custom configuration file paths. Includes precedence logic ensuring --ignore-config overrides --config, error handling for missing/malformed configuration files, and complete unit/integration tests.

[gemini-code-assist]

Code Review

This pull request introduces a --config (or -c) option to the dart_skills_lint CLI, allowing users to specify a custom configuration file path. It updates the configuration loading logic to handle FileSystemException and other potential errors gracefully, and adds comprehensive integration tests to verify the new option's behavior. A review comment suggests improving the error logging for FileSystemException to avoid printing (null) if the file path is not available.

[gemini-code-assist]

If e.path is null, printing (${e.path}) will output (null). It is safer to conditionally append the path only when it is non-null to keep the error message clean.

Suggested change

	} on FileSystemException catch (e) {
	_log.severe('Error: ${e.message} (${e.path})');
	return null;
	} on FileSystemException catch (e) {
	final pathSuffix = e.path != null ? ' (${e.path})' : '';
	_log.severe('Error: ${e.message}$pathSuffix');
	return null;