Nitrogen-vacancy (NV) centre ensembles are leading room-temperature magnetometers, but their sensitivity is usually extracted from a single resonance and relies on external microwave and bias-field hardware. This work proposes a chip-scale NV magnetometer built around a one-dimensional YIG/CoFeB magnonic crystal, in which a single structural defect forms a passive magnonic cavity and integrated permanent magnets supply the bias field. Micromagnetic simulations show a defect-localised spin-wave mode at 2.910 GHz inside the 2.5-3.0 GHz magnonic band gap, 40 MHz from the NV zero-field splitting, with a band structure that matches an independent plane-wave calculation. A Tavis-Cummings/Lindblad model in which the microwave reaches the NV ensemble through this mode predicts a hyperfine-resolved optically detected magnetic resonance spectrum that is strong only within the mode linewidth, and a dispersive homodyne response with one steep edge per ¹⁴N hyperfine line. The master-equation solutions agree with the closed-form homodyne steady state (R² > 0.9999) and with an approximate analytic ODMR theory (peak positions within 0.012 MHz). For a critically coupled readout with room-temperature electronics, combining the three edges gives a projected sensitivity of 93 pT/√(rm Hz), a √3-fold gain over the best single edge. Mapping the sensitivity against the NV-magnon coupling reveals an optimum near g_(rm ens)/2π ≈ 0.2-0.4 MHz, where it reaches 72 pT/√(rm Hz) for a 0.64 MHz spin linewidth and 19 pT/√(rm Hz) for 0.17 MHz, with thermal limits of 6.6 and 1.7 pT/√(rm Hz). The antenna-mode coupling, the NV-magnon coupling and the spin linewidth are therefore the main design levers.