feat(todo): make the parallel in_progress policy configurable

Whether concurrent active tasks are legitimate depends on runtime
concurrency the tool cannot observe, but whether a deployment's agents
ever fan out is knowable at composition time. `allowParallelInProgress`
(default true) therefore replaces the hardcoded policy: the flag moves
the model-facing instruction and the accepted input together, so a
deployment running strictly sequential agents can restore the
single-active discipline from cordis.yml.

The durable-log invariant does not follow the flag. A log written while
parallel work was allowed must still replay after a deployment tightens
the policy, so the invariant stays silent on the active count.

pnpm-lock.yaml

@@ -4727,6 +4727,9 @@ importers:
 
   packages/todo/tool-todo:
     dependencies:
+      schemastery:
+        specifier: ^3.18.0
+        version: 3.18.0
       zod:
         specifier: ^4.4.3
         version: 4.4.3