fix: nil logger panic if command invoked without Cobra hook chain

[jmeridth]

What

c.logger is only initialized inside persistentPreRun, which Cobra calls before each command. If a command is invoked programmatically (e.g., runCmd.Run(runCmd, []string{}) directly), c.logger remains nil and calls like c.logger.Trace(...) panic with a nil pointer dereference.

Why

This is a pre-existing issue from before the dependency injection refactor — the package-level logger global had the same nil exposure. The refactor preserved the behavior but did not add a guard.

Suggested fix

Initialize c.logger to a null logger in NewCLI so it's never nil:

c := &CLI{
    logger: hclog.NewNullLogger(),
    // ...
}

persistentPreRun would still overwrite it with the real logger from config.