Make task schema fields explicit with null sentinels

tests/state.test.ts

@@ -27,7 +27,7 @@ test("adds a nested batch with generated preorder IDs and derived relations", ()
   assert.equal(state.snapshot().nextId, 4);
 });
 
-test("rejects invalid nested parentId atomically", () => {
+test("rejects invalid nested parentId atomically while accepting null sentinels", () => {
   const state = new TaskState();
   state.add([{ name: "Existing", description: "Existing" }]);
   const before = state.snapshot();
@@ -44,6 +44,16 @@ test("rejects invalid nested parentId atomically", () => {
     TaskValidationError,
   );
   assert.deepEqual(state.snapshot(), before);
+
+  const added = state.add([
+    {
+      name: "Nullable parent",
+      parentId: null,
+      children: [{ name: "Implicit child", parentId: null }],
+    },
+  ]);
+  assert.equal(added[0]?.parentId, null);
+  assert.equal(added[1]?.parentId, added[0]?.id);
 });
 
 test("enforces done descendants against the final atomic update state", () => {
@@ -308,23 +318,31 @@ test("loads the latest valid mutation snapshot from a session branch", () => {
   assert.deepEqual(state.list().map((task) => task.name), ["First", "Second"]);
 });
 
-test("supports optional descriptions and rejects invalid provided values", () => {
+test("supports nullable descriptions and reports invalid children as validation errors", () => {
   const state = new TaskState();
   assert.throws(() => state.add([{ name: "Bad\nname", description: "Valid" }]), /single line/);
   assert.throws(() => state.add([{ name: "Blank description", description: "  " }]), /must not be empty/);
+  assert.throws(
+    () => state.add([{ name: "Invalid children", children: "not-an-array" } as never]),
+    /children must be an array/,
+  );
   state.add([
     { name: "No description" },
+    { name: "Null description", description: null },
     { name: "Detailed", description: "Line one\nLine two" },
   ]);
   assert.equal(state.list()[0]?.description, undefined);
-  assert.equal(state.list()[1]?.description, "Line one\nLine two");
+  assert.equal(state.list()[1]?.description, undefined);
+  assert.equal(state.list()[2]?.description, "Line one\nLine two");
 });
 
-test("set append rejects force and still requires at least one task", () => {
+test("set append accepts false force and rejects true force", () => {
   const state = new TaskState();
-  assert.throws(() => state.set("append", [], undefined), /tasks must not be empty/);
-  assert.throws(() => state.set("append", [{ name: "Task" }], false), /force is allowed only in replace mode/);
-  assert.deepEqual(state.snapshot(), { tasks: [], nextId: 1 });
+  assert.throws(() => state.set("append", [], false), /tasks must not be empty/);
+  state.set("append", [{ name: "Task" }], false);
+  const before = state.snapshot();
+  assert.throws(() => state.set("append", [{ name: "Other" }], true), /force must be false in append mode/);
+  assert.deepEqual(state.snapshot(), before);
 });
 
 test("set replace is atomic, guarded, and resets IDs", () => {
@@ -339,7 +357,11 @@ test("set replace is atomic, guarded, and resets IDs", () => {
   );
   assert.deepEqual(state.snapshot(), before);
 
-  const replaced = state.set("replace", [{ name: "New", children: [{ name: "Child" }] }], true);
+  const replaced = state.set(
+    "replace",
+    [{ name: "New", parentId: null, children: [{ name: "Child", parentId: null }] }],
+    true,
+  );
   assert.deepEqual(replaced.map((task) => task.id), [1, 2]);
   assert.deepEqual(state.list().map((task) => task.name), ["New", "Child"]);
   assert.equal(state.snapshot().nextId, 3);