Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2cdot0.6$H$_2$O crystallizes in the $Pbar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$ and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2cdot0.6$H$_2$O enters a magnetic long-range ordered state below $T_text{N}=7.2$~K, and the $mathbf{q}=mathbf{0}$ magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3~K is suppressed to $0.58(2)mu_text{B}$. Our DFT calculations indicate the presence of antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-$frac{1}{2}$ PFFRG analysis shows a dominance of $mathbf{q}=mathbf{0}$ type magnetic correlations, consistent with and indicating proximity to the observed $mathbf{q}=mathbf{0}$ spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet.
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