docs(subagent): state that per-activation knobs are not restored on cold resume

The descriptor deliberately snapshots a curated composition field set
rather than the merge-extensible `AgentOptions`, and it already names the
per-activation exclusions (`outputSchema`). `maxTokens` is the same class
of property — it budgets one activation, and on cold resume there is no
parent to inherit a limit from, so the resumed activation runs under the
deployment defaults. Spell that out in the module contract so the
fallback is a documented decision instead of a silent surprise for
deployments that set explicit child token limits.

packages/subagent/subagent/src/descriptor.ts

@@ -12,6 +12,10 @@
  * omits `subagentDepth` — cold resume trusts the persisted header's
  * `delegationDepth` as the monotone floor — and `outputSchema`, which belongs
  * to one activation's result contract rather than durable child composition.
+ * Per-activation knobs such as `maxTokens` are omitted for the same reason as
+ * `outputSchema`: they budget one activation and, on cold resume, no parent
+ * exists to inherit them from, so the resumed activation runs under the
+ * deployment defaults rather than restoring a stale budget.
  *
  * @module @deepseek-ai/dsh-subagent/descriptor
  */