feat(storage): add parent_task_id migration for micro-stepping

crates/storage/src/migrations.rs

@@ -108,6 +108,10 @@ const MIGRATIONS: &[(i64, &str, &str)] = &[
 
         CREATE INDEX IF NOT EXISTS idx_dictionary_term ON dictionary(term)
     "#),
+    (3, "micro-stepping: parent_task_id on tasks", r#"
+        ALTER TABLE tasks ADD COLUMN parent_task_id TEXT REFERENCES tasks(id) ON DELETE CASCADE;
+        CREATE INDEX IF NOT EXISTS idx_tasks_parent ON tasks(parent_task_id)
+    "#),
 ];
 
 /// Split SQL into individual statements, respecting BEGIN...END trigger blocks.
@@ -207,7 +211,7 @@ mod tests {
             .fetch_one(&pool)
             .await
             .unwrap();
-        assert_eq!(count, 1);
+        assert_eq!(count, 3);
 
         sqlx::query("INSERT INTO settings (key, value) VALUES ('test', 'value')")
             .execute(&pool)
@@ -229,6 +233,40 @@ mod tests {
             .fetch_one(&pool)
             .await
             .unwrap();
-        assert_eq!(count, 1);
+        assert_eq!(count, 3);
+    }
+
+    #[tokio::test]
+    async fn test_parent_task_id_cascade_delete() {
+        let pool = SqlitePoolOptions::new()
+            .connect("sqlite::memory:")
+            .await
+            .unwrap();
+
+        run_migrations(&pool).await.unwrap();
+
+        // Insert parent task
+        sqlx::query("INSERT INTO tasks (id, text) VALUES ('parent-1', 'Parent task')")
+            .execute(&pool)
+            .await
+            .unwrap();
+
+        // Insert child task with parent_task_id
+        sqlx::query("INSERT INTO tasks (id, text, parent_task_id) VALUES ('child-1', 'Child task', 'parent-1')")
+            .execute(&pool)
+            .await
+            .unwrap();
+
+        // Delete parent — child should cascade
+        sqlx::query("DELETE FROM tasks WHERE id = 'parent-1'")
+            .execute(&pool)
+            .await
+            .unwrap();
+
+        let remaining: i64 = sqlx::query_scalar("SELECT COUNT(*) FROM tasks")
+            .fetch_one(&pool)
+            .await
+            .unwrap();
+        assert_eq!(remaining, 0, "cascade delete should remove child when parent is deleted");
     }
 }