refactor(config): remove the unwired config module

Config::load was never called: the DNS-override story it carried
contradicts the actual resolver flow, and nothing in the CLI or
snap advertised a key. Delete the module and every mention of the
ecrou.yaml file in SPEC and the snap; the XDG_CONFIG_HOME redirect
in the command chain served only that file.

SPEC.md

@@ -343,8 +343,6 @@ tool.
 └── <...>.provisioned                   # hot-cell marker
 ```
 
-`~/.config/ecrou.yaml` carries over (DNS overrides today).
-
 ## Snap packaging
 
 Confinement: strict. Plugs: `network`, `home`, `kvm`. Manual