Genome-Wide Identification of the β-Amylase (BAM) Gene Family in Senna tora and Seven Other Legumes: Characterization of StoBAM6 as a Cold-Responsive Candidate Gene and a Potential Target of StoCBF2

β-Amylases (BAMs) hydrolyze starch and contribute to abiotic stress acclimation, with AtBAM3 mediating most transitory starch degradation in Arabidopsis; however, legume BAM family evolution and the effect of whole-genome duplications remain unexplored. We identified 92 BAM genes across eight legume genomes and classified 111 BAM proteins (including Arabidopsis and rice references) into four groups, with subgroup II-B containing all AtBAM3 orthologs. In S. tora, StoBAM6 and StoBAM10 formed a collinear paralogous pair unlikely to derive from the Papilionoideae-specific duplication. Under 4 °C treatment, StoBAM6 was induced within 1 h and stayed elevated through 24 h, whereas StoBAM10 rose only transiently at 3 h. StoBAM6 expression correlated positively with leaf β-amylase activity (r = 0.7557, p < 0.001) and negatively with starch content (r = −0.7343, p < 0.001), whereas StoBAM10 showed no significant correlation with β-amylase activity (r = 0.7728, p = 0.75) or starch content (r = −0.0685, p = 0.77). StoCBF2 shared StoBAM6’s cold-induced expression profile; yeast one-hybrid assays showed that StoCBF2 binds a StoBAM6 promoter fragment harboring the DRE-core motif. These results indicate regulatory divergence after duplication, identify StoCBF2 as a candidate upstream regulator of StoBAM6, and provide a working model for cold-responsive starch metabolism in S. tora.