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, and flag metadata uses FlagOption / Command.FlagOptions. This branch keeps the core help surface string-based while making customization clearer:

• Command.Help func(*Command) string replaces UsageFunc.
• cli.Help(*Command) string renders help for the resolved command.
• usage is optional for structured composition via Document, New, Help, Text, Lines, List, Flags, and Commands.
• State.Cmd exposes the resolved command inside Exec.
• cli.UsageErrorf marks invalid command-line usage from Exec; Run prints help before returning the underlying error.
• FlagConfig / Command.FlagConfigs replace FlagOption / Command.FlagOptions.

Why: the default path stays small and automatic, while 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"
}

// 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()
}

// 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.
• Remove DefaultUsage and Usage; use cli.Help(cmd).
• Rename FlagOption to FlagConfig and Command.FlagOptions to Command.FlagConfigs.

Closes #4, #14, #15.