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Detailed studies of our Local Group benchmark our understanding of galaxy formation. Stellar metallicities are key tracers of the baryonic astrophysics shaping galactic properties, but they are challenging to measure in distant and faint galaxies that are pushing push our understanding of dwarf galaxy formation to new regimes in luminosity, star formation history, and environment. For my thesis, I present hundreds of new stellar metallicities in faint, Local Group dwarf galaxies, measured through a novel use of HST narrowband Ca H&K imaging. Our imaging includes: 1) 463 stars in 13 ultra-faint dwarf galaxies (UFDs) around the Milky Way, which effectively doubles the number of stellar metallicities in all known UFDs, b) 374 stellar metallicities in the quenched field dwarf galaxy Tucana (Mv = -8.8, D = 1 Mpc), a factor of ~7 increase over literature spectroscopy, and c) 286 stellar metallicities in two M31 dwarfs And XVI and And XXVIII. I will present highlights from the wide range of science cases enabled by our data, which include: 1) chemical evolution modeling to put novel constraints on the baryon cycle in UFDs, 2) new metallicity benchmarks for cosmological simulations of the faintest galaxies, 3) high-fidelity metallicity gradients that constrain stellar feedback and DM core formation models in dwarf galaxies. I conclude with a discussion on the immense scientific potential for narrowband Ca H&K imaging to transform stellar metallicity studies at the edge of the LG and beyond.