feat(egress): add preserve_auth flag to pass agent Authorization through

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

bot_bottle/egress.py

@@ -146,6 +146,7 @@ def egress_manifest_routes(
             outbound_detectors=r.OutboundDetectors,
             inbound_detectors=r.InboundDetectors,
             outbound_on_match=r.OutboundOnMatch,
+            preserve_auth=r.PreserveAuth,
         ))
     return tuple(out)