Review point: the co-located defaults are an auto-deploy special case and
belong with the other ones, not inside each option's own default. An
option should describe itself; the mode describes what a deployment shape
implies. local-defaults.nix already says exactly this in its header.
Options now default to empty (= unset, which the assertions refuse), and
enableAllLocalDefaults fills in loopback + the minted secret path.
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