feat: add LegacyBoolHelp to restore pre-v1.0.6 bool help formatting

[leno23]

Summary

Fixes #454

Since #410, bool flags include [=true|false] and (default true/false) in help output. Some projects relied on the previous compact formatting for 80-column layouts.

This adds FlagSet.LegacyBoolHelp. When enabled, bool and boolfunc flags omit the syntax hint and default value in help text while keeping current behavior as the default.

Test plan

• Added TestLegacyBoolHelp and TestLegacyBoolHelpBoolFunc
• go test ./...

Made with Cursor

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