make openapi-generator target references openapi-public.yaml, which doesn't exist in the repo #4

Context

PR #1 added:

openapi-generator:
	openapi-generator-cli generate -i openapi-public.yaml -g go -o ./internal/workflow_client -c .openapi-generator.yaml

openapi-public.yaml isn't checked into the repository and isn't produced
by any other target. Per PR #1 discussion, this is intentional — the spec
file itself shouldn't be public — but that leaves the target broken for
anyone who runs it from a clean checkout, with no explanation of where the
input file is supposed to come from.

Ask

Pick one:

• Remove the target from the public Makefile if it genuinely can't work
  outside an internal environment, and document the internal regeneration
  process wherever that lives instead.
• Or, if the spec can be fetched from somewhere (internal artifact store,
  URL, etc.), have the target retrieve it (or document the manual step) so
  make openapi-generator is actually runnable as written.

Follow-up from PR #1 review.