feat: make all cli flags available as env variables (opt-in)

[mfridman]

This might not be necessary since it's just a few helper functions most users can stub out in their code, e.g.,

func getFlagOrEnv(s *cli.State, name string, envPrefix string) (string, error) {
	envName := strings.ToUpper(name)
	val := cmp.Or(
		cli.GetFlag[string](s, name),
		os.Getenv(envPrefix+envName),
	)
	if val == "" {
		return "", fmt.Errorf("must provide --%s flag or set %s environment variable", name, envName)
	}
	return val, nil
}

We could optionally bake this logic into the cli library or add a cli.GetFlagOrEnv helper function.

[mfridman]

Priority: CLI args > env vars > flag defaults

Two opt-in mechanisms that work together:

Per-flag via FlagMetadata.Env -- explicit, no magic. Useful when env var names don't follow a pattern (e.g., DATABASE_URL).

FlagsMetadata: []cli.FlagMetadata{
    {Name: "db-url", Env: "DATABASE_URL", Required: true},
    {Name: "verbose", Env: "VERBOSE"},
},

Auto-mapping via FromEnv -- maps all flags automatically using a prefix. Unlike ff, the subcommand name is included, so myapp serve --port maps to MYAPP_SERVE_PORT instead of just MYAPP_PORT, avoiding collisions when subcommands share flag names.

cli.Parse(root, os.Args[1:], cli.FromEnv("MYAPP"))

If a flag has an explicit Env, it takes precedence over the auto-generated name.