Summary

• Deprecate config.LoadConfig and the config.Paths helper in favour of the ConfigLoader plugin, centralising all scoped config resolution through ConfigSource.
• Add three ConfigSource implementations: Disk (host filesystem only), Environment (respects GIT_CONFIG_* and XDG_CONFIG_HOME), and Auto (merges both, matching default Git behaviour).
• Update EXTENDING.md with ConfigLoader plugin documentation.

Opt-in to Git's behaviour when using go-git

Parts of go-git that depend on system or global configuration began using the ConfigLoader plugin with the introduction of #1860. By default, config.Configs for both System and Global are initialized as empty. This behaviour is intentional, as it prevents unintended side effects in applications that use go-git to automate operations outside of a user-specific context.

For applications that need go-git to mimic the behavior of the standard git CLI, this can be enabled by registering the Auto ConfigSource:

func init() {
    plugin.Register(plugin.ConfigLoader(), func() plugin.ConfigSource {
        return xconfig.NewAuto()
    })
}

Fixes #395 #1701 #1849.
Supersedes #1702.
Follows-up from #1860.