hive-agent: extra web proxies forward every repeated response header

Both proxy paths copied upstream response headers with HeaderMap::insert,
which replaces earlier values of the same name, so an upstream sending two
Set-Cookie (or Link, Vary, ...) headers reached the browser with only the
last one. Use append in both loops. Tests cover the http and unix: paths.

docs/web-ui/agent.md

@@ -380,7 +380,8 @@ shaped).
   `{"<name>": "<upstream_url>"}`). Each entry mounts a transparent
   reverse-proxy at `/extra/<name>/` that forwards every request (method,
   headers, body) to the configured upstream, strips hop-by-hop headers
-  on both sides, and buffers the full response body (MVP — SSE
+  on both sides (repeated response headers such as two `Set-Cookie` lines
+  are all forwarded), and buffers the full response body (MVP — SSE
   connections will appear as one large response rather than streaming).
   The `/extra/` namespace ensures user-declared proxies can never
   conflict with native agent endpoints. Upstream values are either an