Merge branch 'web2-todo' into feat/todo-multi-in-progress

Stack the parallel-in_progress change on the web todo display (#497): the
GUI is now the surface where several active items are visible, so the two
land as a chain rather than colliding on tool-todo at merge time.

Conflicts combined rather than resolved to one side: tool-todo keeps this
branch's parallel-allowing validation AND web2-todo's additionalProperties
unknown-key rejection, in src/index.ts and both README sides; the spec
drops web2-todo's 'two in_progress' rejection case and keeps its unknown-key
case; the two headless advanced-toolchain session fixtures keep this
branch's parallel transcripts.

packages/todo/tool-todo/src/index.ts

@@ -32,7 +32,10 @@ const DESCRIPTION =
  * Validate the value constraints the ParameterSchemaSpec can't express and build the canonical {@link
  * TodoItem}[]: trimmed non-empty unique content. Any number of items may be in_progress —
  * parallel work (subagents, background commands) legitimately runs several tasks at once. The
- * registry has already enforced the status enum; the cast below records that guarantee.
+ * registry has already enforced the status enum and rejected unknown item keys
+ * (`additionalProperties: false` — the logged snapshot must equal what the model believes it
+ * wrote, so a nested/extended item shape fails loud at the schema boundary instead of silently
+ * flattening); the cast below records that guarantee.
  */
 function toTodoList(raw: { content: string; status: string }[]): TodoItem[] {
   const todos: TodoItem[] = []
@@ -63,7 +66,7 @@ export function apply(ctx: Context): void {
         description: 'The COMPLETE task list, replacing any previous list.',
         items: {
           type: 'object',
-          additionalProperties: true,
+          additionalProperties: false,
           properties: {
             content: { type: 'string', required: true, description: 'What the task is — a short imperative line.' },
             status: {