fix(proxy): enforce sandbox — proxy_net internal, cf-proxy dual-homed

Before this change HTTPS_PROXY was a polite recommendation, not a
boundary: proxy_net was a regular bridge with NAT, so anything inside
the claude container could open a direct socket to the internet and
silently bypass squid's allowlist.

Now proxy_net is internal (no NAT, no outbound). cf-proxy joins a
second bridge egress_net to dial the Cloudflare edge. claude stays on
proxy_net only, so its only route off-host is through cf-proxy →
CF Access → tunnel → squid → allowlisted CONNECTs.

Verified: from a sibling container on proxy_net, curl https://1.1.1.1
fails immediately ("Could not connect to server"); the same container
through cf-proxy gets HTTP 401 from api.anthropic.com (the smoke-test
success code).

proxy/compose.yml

@@ -1,12 +1,20 @@
-# Run Claude Code through proxy.fhirworx.io from inside containers.
+# Run Claude Code in a sandboxed container whose only outside path is the
+# allowlisted forward proxy at proxy.fhirworx.io.
 #
-# Two services on a self-contained 192.168.10.0/24 bridge:
-#   cf-proxy  cloudflared access TCP shim — terminates the Cloudflare tunnel
-#             on 192.168.10.2:18443. Equivalent to the systemd --user unit
-#             that dev/scripts/setup-proxy-client.sh installs on a WSL host.
-#   claude    Claude Code CLI. Reaches cloudflared via the bridge; extra_hosts
-#             pins proxy.fhirworx.io -> 192.168.10.2 so TLS SNI/cert match
-#             without poisoning cf-proxy's own DNS view.
+# Network topology (the point of this compose):
+#   proxy_net  internal bridge, 192.168.10.0/24. No NAT, no internet egress.
+#              claude lives only here, so it physically cannot reach anything
+#              outside the host except by going through cf-proxy.
+#   egress_net regular bridge. Only cf-proxy is dual-homed: it sits on
+#              proxy_net to be reachable by claude, and on egress_net so
+#              cloudflared can dial the Cloudflare edge.
+#
+# Services:
+#   cf-proxy  cloudflared access TCP shim. Terminates the Cloudflare tunnel
+#             at 192.168.10.2:18443. The only door out for claude.
+#   claude    Claude Code CLI. HTTPS_PROXY points at proxy.fhirworx.io:18443,
+#             which extra_hosts pins to 192.168.10.2 so the TLS SNI/cert
+#             match without poisoning cf-proxy's own DNS view.
 #
 # Usage:
 #   cp .env.example .env && $EDITOR .env       # set PROXY_PASSWORD and
@@ -18,7 +26,7 @@
 # - claude-home is a named volume so login state persists across runs.
 # - Bind mounts use identical host/container paths so Claude never sees a
 #   translated path: /home/care/acoharmony, /opt/s3/data/workspace,
-#   /opt/s3/data/notebooks.
+#   /opt/s3/data/notebooks. Claude only has filesystem access to these.
 
 services:
   cf-proxy:
@@ -31,6 +39,7 @@ services:
     networks:
       proxy_net:
         ipv4_address: 192.168.10.2
+      egress_net: {}
     restart: unless-stopped
 
   claude:
@@ -58,9 +67,12 @@ services:
 networks:
   proxy_net:
     driver: bridge
+    internal: true
     ipam:
       config:
         - subnet: 192.168.10.0/24
+  egress_net:
+    driver: bridge
 
 volumes:
   claude-home: