Refactor help API

[mfridman]

This PR reshapes help customization and usage-error handling against the API on main.

On main, custom help is Command.UsageFunc func(*Command) string, default help lives behind DefaultUsage, help detection re-exports flag.ErrHelp as cli.ErrHelp, command descriptions use ShortHelp, and flag metadata uses FlagOption / Command.FlagOptions. This branch keeps core help automatic and string-based, while moving manual rendering and structured composition to the optional usage package:

• Command.Help func(*Command) string replaces UsageFunc for custom help.
• Command.Description replaces ShortHelp for command descriptions.
• --help and cli.UsageErrorf still print command help automatically.
• Parse returns flag.ErrHelp when callers handle help themselves.
• usage.Help(cmd) renders the same help text ParseAndRun would print for --help.
• usage.New(cmd) builds a composable usage.Document for adding sections like Examples.
• State.Cmd exposes the resolved command inside Exec.
• FlagConfig / Command.FlagConfigs replace FlagOption / Command.FlagOptions.

Why: the default path stays small, automatic, and out of the way. Callers that want richer help can opt into usage without making structured documents part of the core API.

Examples:

// main
cmd.UsageFunc = func(c *cli.Command) string {
    return "custom help"
}

// this PR
cmd.Help = func(c *cli.Command) string {
    return "custom help"
}

// main
ShortHelp: "print a greeting",

// this PR
Description: "print a greeting",

// keep the default help and add examples
cmd.Help = func(c *cli.Command) string {
    doc := usage.New(c)
    doc = append(doc, usage.Lines("Examples:", "greet margo"))
    return doc.String()
}

// handle help manually after cli.Parse
if err := cli.Parse(root, args); err != nil {
    if errors.Is(err, flag.ErrHelp) {
        fmt.Fprintln(stdout, usage.Help(root))
        return nil
    }
    return err
}

// print command help for invalid args discovered in Exec
Exec: func(ctx context.Context, s *cli.State) error {
    if len(s.Args) == 0 {
        return cli.UsageErrorf("must supply a name")
    }
    return nil
},

Breaking changes:

• Remove Command.UsageFunc; use Command.Help.
• Rename Command.ShortHelp to Command.Description.
• Remove DefaultUsage and the top-level Usage function; use usage.Help(cmd) or usage.New(cmd).
• Remove cli.ErrHelp; use flag.ErrHelp.
• Rename FlagOption to FlagConfig and Command.FlagOptions to Command.FlagConfigs.

Closes #4, #14, #15.