  The scalable application of quantum information science will stand on
reproducible and controllable high-coherence quantum bits (qubits). Here, we
revisit the design and fabrication of the superconducting flux qubit, achieving
a planar device with broad frequency tunability, strong anharmonicity, high
reproducibility, and relaxation times in excess of $40\,\mu$s at its
flux-insensitive point. Qubit relaxation times $T_1$ across 22 qubits are
consistently matched with a single model involving resonator loss, ohmic charge
noise, and 1/f flux noise, a noise source previously considered primarily in
the context of dephasing. We furthermore demonstrate that qubit dephasing at
the flux-insensitive point is dominated by residual thermal photons in the
readout resonator. The resulting photon shot noise is mitigated using a
dynamical decoupling protocol, resulting in $T_2\approx 85\,\mu$s,
approximately the $2T_1$ limit. In addition to realizing an improved flux
qubit, our results uniquely identify photon shot noise as limiting $T_2$ in
contemporary qubits based on transverse qubit-resonator interaction.
