Act as a seasoned novelist with a decade of experience in mystery and psychological thrillers. Craft a short story (800-1,200 words) about a struggling writer who discovers that their unfinished manuscripts are being mysteriously completed overnight—with eerie precision and in their own writing style. The protagonist must unravel whether this is the work of a [SUPERNATURAL ENTITY], a [TECH-SAVVY INTRUDER], or a [DISSOCIATIVE MEMORY LAPSE]. Include atmospheric details, a twist ending, and subtle clues that hint at the true nature of the 'co-author.' Focus on themes of identity, creativity, and obsession.