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.

packages/todo/tool-todo/package.json

@@ -32,6 +32,7 @@
   ],
   "license": "BSD-3-Clause",
   "dependencies": {
+    "schemastery": "^3.18.0",
     "zod": "^4.4.3"
   },
   "peerDependencies": {