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

tests/unit/test_egress_addon_request_flow.py

@@ -413,6 +413,28 @@ class TestAuthInjection(unittest.TestCase):
         assert flow.response is not None
         self.assertEqual(403, flow.response.status_code)
 
+    def test_preserve_auth_passes_agent_token_through(self) -> None:
+        route = Route(host="registry-1.docker.io", preserve_auth=True)
+        addon = _addon(Config(routes=(route,)))
+        flow = _Flow(_Request(
+            host="registry-1.docker.io",
+            headers={"authorization": "Bearer agent-registry-token"},
+        ))
+        _run_request(addon, flow)
+        self.assertEqual("Bearer agent-registry-token", flow.request.headers.get("authorization"))
+        self.assertIsNone(flow.response)
+
+    def test_default_route_strips_agent_auth(self) -> None:
+        route = Route(host="registry-1.docker.io")
+        addon = _addon(Config(routes=(route,)))
+        flow = _Flow(_Request(
+            host="registry-1.docker.io",
+            headers={"authorization": "Bearer agent-registry-token"},
+        ))
+        _run_request(addon, flow)
+        self.assertIsNone(flow.request.headers.get("authorization"))
+        self.assertIsNone(flow.response)
+
 
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