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/tests/tool-todo.spec.ts

@@ -143,6 +143,7 @@ describe('dsh-tool-todo', () => {
   it.each([
     { label: 'empty content', todos: [{ content: '   ', status: 'pending' }], fragment: 'non-empty' },
     { label: 'duplicate content', todos: [{ content: 'dup', status: 'pending' }, { content: 'dup', status: 'completed' }], fragment: 'duplicate' },
+    { label: 'unknown item keys', todos: [{ content: 'a', status: 'pending', children: [] }], fragment: 'not a declared property' },
   ])('rejects $label as an isError result', async ({ todos, fragment }) => {
     const ctx = await setup()
     const result = await callTodo(ctx, { todos })