feat(webchat): support image attachments

docs/plans/state.json

@@ -444,8 +444,15 @@
               "files_modified": ["src/channels/whatsapp/adapter.ts"]
             },
             "webchat": {
-              "status": "deferred",
-              "description": "Requires gateway protocol update for WebSocket attachment messages"
+              "status": "completed",
+              "description": "Webchat UI can send image attachments via gateway agent.send attachments parameter; agent handler accepts attachment-only messages",
+              "files_modified": [
+                "src/gateway/ui/pages/chat.js",
+                "src/gateway/ui/chat.html",
+                "src/gateway/ui/style.css",
+                "src/gateway/handlers/agent.ts",
+                "src/gateway/handlers/handlers.test.ts"
+              ]
             }
           }
         }
@@ -1827,7 +1834,7 @@
   },
 
   "overall_progress": {
-    "total_test_count": 1617,
+    "total_test_count": 1625,
     "all_tests_passing": true,
     "p0_completion": "3/3 (100%)",
     "p1_completion": "4/4 (100%)",