`NodeId` has been globally unique across DAGs since #2801, so the dag id
carried no information the node id didn't. The parameter was already
underscore-prefixed as unused, but still populated by `claim_ready`, carried
through the scheduler's mpsc on every `Claim`, and passed at the call site —
three layers of plumbing feeding a dead argument.
Removing it surfaced four more dead things it had been keeping alive:
`settle_approval_tail`, `settle_rebuild_tail` and `drain_meta_syncs` each took
a dag id they only forwarded to `complete_node`, and one `submit` binding was
never read. Those are deleted rather than underscore-prefixed — prefixing is
what let the original argument survive this long.
`Claim.dag_id` stays: it has live consumers in the tracing spans,
`append_subgraph`'s container guard, `Ctx` for the build-log link,
`first_error`, and the approval-deploy context.
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