Support splitting subcommands across multiple classes with the same @Command group

[dblevins]

Summary

When two classes share the same @Command group name, only the last class registered is accessible. The earlier class's subcommands are silently overwritten.

For example, given:

@Command("auth")
public class LoginCommand {
    @Command("login")
    public void login(final String name) {}
}

@Command("auth")
public class RefreshCommand {
    @Command("refresh")
    public void refresh(final String name) {}
}

Currently auth refresh Foo works but auth login Foo fails with "No such sub-command: login" because RefreshCommand overwrites LoginCommand in the command map.

Expected Behavior

Both auth login Foo and auth refresh Foo should work. When multiple classes share the same @Command group name, their subcommands should be merged into a single CmdGroup.

This should also support overloaded subcommands across classes — two classes defining the same subcommand name with different signatures should merge into an OverloadedCmdMethod.

Implementation

The fix involves two changes:

• CmdGroup — Add a merge(CmdGroup) method that combines subcommands from another group, handling all combinations of CmdMethod and OverloadedCmdMethod
• Main.processClass() — When inserting a CmdGroup, check if one already exists with the same name and merge instead of overwriting