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.

docs/tool-catalog.md

@@ -1137,7 +1137,7 @@ Record and update a structured task list for the current work. Send the ENTIRE l
       "description": "The COMPLETE task list, replacing any previous list.",
       "items": {
         "type": "object",
-        "additionalProperties": true,
+        "additionalProperties": false,
         "properties": {
           "content": {
             "type": "string",