Problem
Large backlog items (XL and above) are too big for a single agent dev session. We need a formal decomposition pattern so that:
1. Items over a size threshold (configurable, default XL) must be decomposed into smaller sub-items before work begins.
2. Sub-items become the actual units of work — each completable in one agent session.
3. The parent item tracks overall progress via its children.
Design considerations
Could be enforced via `sw_validate` or as a workflow gate (e.g., `sw_claim` refuses XL+ items that haven't been decomposed).
Parent/child relationship type needed (or reuse `tracks`/`tracked_by`).
`sw_pull_next` should skip undecomposed XL+ items and instead recommend their children.
Board view should show rollup progress for parent items.
Threshold should be configurable in board.yaml or KB config.