In "A Poison Tree," what is the literal meaning of "And into my garden stole"? How did the persona's enemy enter his garden?

Literally, the persona's enemy "stole" into the garden, which means "crept" or "sneaked" in quietly. Literally, therefore, this line means the enemy sneaks into the persona's garden. We also learn that it is very dark outside, for the poem says "when the night had veiled the pole." The "pole" most likely refers to the star Polaris, the North star, which is perhaps on this particular night obscured by clouds. 

However, "stole" is, beyond its literal meaning, also a double entendre, a word with more than one meaning. Stole does mean creep or sneak and literally the enemy sneaks in. But stole also is the past tense for steal in the sense of rob, so there is a suggestion or connotation of the enemy not only creeping in but creeping in illicitly to steal.