Adds a per-route boolean field preserve_auth (default false) that skips
the gateway's Authorization header stripping for that host. Intended for
registry endpoints like Docker Hub (registry-1.docker.io) and GHCR
(ghcr.io) where the agent must supply its own per-scope bearer token.
Threaded through ManifestEgressRoute → EgressRoute → Route, serialized
in route_to_yaml_dict, and parsed in parse_routes. The strip at
egress_addon.py now checks route.preserve_auth before popping the header.
Closes#392