Manage a persistent stack of paused work — plans, progress, and loose ends — so it can be resumed in a later conversation. Use when the user says /stack-it, or when they want to pause current work to handle an interruption (bug fix, refactoring) and come back later. Supports push, pop, show, list, and drop subcommands. Also trigger when the user says things like "stack this", "save this for later", "park this work", or "resume stacked work".
