Every reorderable surface (repos, worktrees, sessions, pins) now drops in a
single ReorderableList that owns its own DndContext, so a drag only ever
collides with that list's own items — nesting "just works" without leaking
into the lists around or inside it. This replaces the shared DndContext +
id-prefix dispatch (parent:/group:) whose closestCenter collisions resolved
to a different-typed droppable and silently no-op'd worktree/repo drags.
- Delete groupDndId/parentDndId/parse* helpers and the monolithic
handleAgentDragEnd/handlePinnedDragEnd; each list persists its new id order
via a direct typed write (reorderParents/reorderWorktree/reorderSessions/
reorderPinned).
- Sessions inside repos/worktrees are date-ordered and static (no drag),
matching the "never reorder on new messages" rule.
- Add setPinnedSessionOrder; drop now-unused reorderPinnedSession.
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