Step 3 of the hive-gateway.nix vhost cleanup. Lift the /agent/ catch-all
(+ its two internal error-page targets) and the catch-all dashboard proxy
out of the inline //-chain into agentLocations / dashboardProxyLocation
bindings. The default _ server's locations now read as a flat composition
of named groups — matrixRedirectLocations // wellKnownLocations //
agentLocations // dashboardProxyLocation // <auth optionalAttrs> — instead
of a deep nested literal. The auth-401 group stays inline (a self-contained
lib.optionalAttrs already).
Pure readability refactor, eval-identical: generated virtualHosts toJSON is
byte-identical before/after (8888 bytes, diff empty).
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