I dwell in Possibility
Possibility vs. Prose
Dickinson sets up poetry against prose fiction—'Possibility' is her metaphor for the poetic imagination. She's claiming poetry has more room, more openings.
Cedars as chambers
The comparison is structural—cedars are tall, many-branched, dense. 'Impregnable of eye' means you can't see into them, can't count their rooms.
The gables
Gables are roof peaks—she's claiming the sky itself as her ceiling. The house of poetry has no actual roof, it opens infinitely upward.
The fairest visitors
Likely the Muses, or poetic inspiration itself. In her house metaphor, these are the guests poetry receives that prose cannot.
Narrow hands
The physical smallness (her actual hands, her limited life in Amherst) contrasted with what they reach for. The gesture is both humble and ambitious.