Emit goodbye POST when diagnostics push toggled off #226

Job	Run time
mounted image upgrade (legacy contract)	1m 12s
image build mode	14s
webconfig drift	1m 8s
unit tests	3m 11s
script upgrade (stable main)	49s
script install (ubuntu:24, bundle)	47s
script upgrade (pre-schema-split pin)	46s
mounted image upgrade (new contract)	1m 9s
script install (ubuntu:24, bootstrap)	45s
script install (debian:13, bootstrap)	1m 17s
lint	27s
script install (debian:13, bundle)	44s
	12m 29s

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