Show usage on positional-arg errors

entire repo list (and any command with required positional args) printed only the bare cobra message ("accepts 1 arg(s), received 0"), leaving the user to look up the right invocation.

Extend main.go's error-routing switch (which already shows usage for unknown-command / unknown-flag) with an arg-count branch. Switch from ExecuteContext to ExecuteContextC so the deepest matched command is available — that's the one whose Args validator failed and owns the right usage block — and route it through the existing showSuggestion path rather than the root command's usage.

The matcher is a substring check on "arg(s)": cobra's stock validators (ExactArgs, NoArgs, MinimumNArgs, MaximumNArgs, RangeArgs) all surface that substring, and it doesn't appear in cobra's other error paths or the cli's own errors.