decompose-into-slices

Typical invocation points:

• After write-milestone-brief (or after a discuss phase that produced a brief)
• When a roadmap exists but slices are too thick, too few, or poorly ordered
• When exporting the plan for external collaborators (GitHub issues)

<core_principle> VERTICAL, NOT HORIZONTAL. A slice that adds "schema + API + UI + tests for feature X on one narrow path" is vertical. A slice that adds "all schemas for all features" is horizontal and is wrong. Horizontal slices destroy the demoability property that makes slice completion meaningful.

MANY THIN, NOT FEW THICK. If a slice could be split into two demoable pieces, split it. Thin slices retire risk earlier, parallelize better, and give the user faster feedback on whether the direction is right.

DEPENDENCY GRAPH, NOT LINEAR LIST. Slices that don't depend on each other should be marked that way — depends:[] means the slice can start immediately. The GSD engine uses this to parallelize.