feat(FN-2383): persist task priority across core storage

- Add task-priority contract, normalization helpers, and exports in @fusion/core types/index
- Store task priority in SQLite and migrate existing databases with default values
- Update task store behavior and sorting tests to preserve and order by persisted priority
- Add migration/regression coverage for archived tasks and refresh storage/task-management docs

docs/task-management.md

@@ -193,7 +193,7 @@ Behavior:
 
 Archive entries preserve key metadata needed for restoration, including:
 
-- `id`, `title`, `description`, `column`
+- `id`, `title`, `description`, `priority`, `column`
 - `dependencies`, `steps`, `currentStep`
 - `size`, `reviewLevel`, `prInfo`, `issueInfo`
 - `attachments` metadata