03 — Subtasks: arbitrary-depth nesting, ordering, cascade-down completion

What to build: Any Task can be given Subtasks, to arbitrary depth; Lists themselves never nest. The List view renders the task tree. Subtask order is meaningful (it is the intended execution order) and the user can reorder siblings. Completing a Task completes its entire subtree (cascade down); completing all Subtasks never auto-completes the parent (no cascade up). Reopening semantics must leave the tree in a sensible state.

Blocked by: 02 — Flat Tasks in a List.

Status: done (commit 61c0a81, 2026-08-04)

☐ User can add a Subtask under any Task, at any depth
☐ List view renders the nested tree legibly on both desktop and phone widths
☐ User can reorder sibling Subtasks; the order persists
☐ Completing a parent marks every open descendant completed
☐ Completing the last open Subtask leaves the parent open
☐ Hidden-completed toggle works correctly within the tree
☐ Tests cover the "Renovate bathroom" scenario from CONTEXT.md: cascade down, never up