Updates the node-wide LLM cost budget the governor enforces on every
invocation. Applies to chat and to workflow applications that invoke
the LLM — this is the only enforcement layer that catches non-chat
callers (per-user chat quotas don't apply to workflow apps).

Permissions: Node owner only.

Note: the governor caches SSM values for ~60 seconds, so changes take
up to a minute to take effect on in-flight invocations.