Clause 2 of #3202, reading 1 (mara: "the all local stuff and swarm
services auto conf belong in those mods, not spread all over").
`gateway.localHostsEntry` is the gateway's only local-deployment knob —
`openFirewall` is about EXTERNAL exposure, `tls.acme` needs a public DNS
name, `hsts` is a hardening choice. It is now asserted by the mode in
local-defaults.nix, beside the three swarm toggles, instead of being the
one all-local implication an operator still had to know about.
`mkDefault`, so "all local except this" still needs no new option.
⚠️ The non-obvious half: this does NOT change what CONTAINERS resolve.
dnsmasq sets `no-hosts = true` unconditionally, so agents keep getting
the bridge IP from the authoritative `address=` rules rather than the
host's 127.0.0.1 — which would point every agent at its own netns. That
guard already existing is what makes this safe to default on; without it
this one line would break every agent's access to the forge.
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