environment.nix explains why HYPERHIVE_OTEL_HEADERS_CREDENTIAL is not
emitted by naming the machinery that used to consume it -- machinery this
PR deletes. Left as written it would describe a removed nspawn credential
and a removed oneshot as if both still ran, in the same paragraph that
justifies withholding the variable.
That is this PR's own defect inverted: it exists so an auditor asking
'can an agent obtain the OTEL token?' finds nothing misleading, and a
comment claiming live delivery machinery is exactly the kind of thing
that costs an auditor a reconstruction.
Keeps the rule rather than the history -- one holder, on the host -- since
that is the part still worth reading.
72b2a0357b