Expand Spec Incremental

Drive a spec that is too broad to build in one pass, ONE increment at a time, with the user in the loop. Use when a product/architecture/technical spec (or a rough plan) covers more than should be executed as a single change, and the user wants to slice it into scoped increments and land them incrementally. For each increment this skill: scopes the next slice with the user, GENERATES a focused, self-contained increment plan via the Fable planner, EXECUTES that plan's tasks sequentially as a todo list, and posts lifecycle updates to #agent-updates — then checkpoints with the user and moves to the next slice. It deliberately runs NO adversarial review: the human reviewing …