Update README with capabilities negotiation flow

- Add capabilities negotiation to Features list
- Document connection flow (5 steps)
- Add capabilities_response message format
- Add request_capabilities message format
- Update JavaScript example with capabilities handling
- Update Python example with capabilities handling

README.md

@@ -14,6 +14,7 @@ Custom WebSocket-based messaging channel plugin for OpenClaw. Enables custom mes
   - Reactions
   - Threads
   - Typing indicators
+- 🔄 **Capabilities Negotiation** - Clients report their capabilities on connect
 - 🛡️ **DM Security** - Three policies: `allowlist`, `open`, `closed`
 - 🔗 **Threading** - Full support for threaded conversations
 - 🤝 **Pairing Flow** - Text-based pairing with code exchange
@@ -87,9 +88,41 @@ const ws = new WebSocket('ws://localhost:8765/ws', {
 });
 ```
 
+### Connection Flow
+
+1. **Client connects** → Server authenticates via API key
+2. **Server sends handshake** → Server capabilities (maximum allowed)
+3. **Server requests capabilities** → Asks client what it supports
+4. **Client responds** → Client capabilities + metadata
+5. **Effective capabilities** → Server uses intersection of both
+6. **Messaging begins** → Only supported message types allowed
+
 ### Message Format
 
-**Client → Server:**
+#### Client → Server
+
+**Capabilities Response (required after handshake):**
+```json
+{
+  "type": "capabilities_response",
+  "capabilities": {
+    "text": true,
+    "images": true,
+    "files": false,
+    "voice": false,
+    "reactions": false,
+    "threads": true,
+    "typing": true
+  },
+  "metadata": {
+    "clientName": "My App",
+    "version": "1.0.0",
+    "platform": "custom"
+  }
+}
+```
+
+**Text Message:**
 ```json
 {
   "type": "message",
@@ -101,7 +134,9 @@ const ws = new WebSocket('ws://localhost:8765/ws', {
 }
 ```
 
-**Server → Client (Handshake):**
+#### Server → Client
+
+**Handshake:**
 ```json
 {
   "type": "handshake",
@@ -109,19 +144,27 @@ const ws = new WebSocket('ws://localhost:8765/ws', {
   "capabilities": {
     "text": true,
     "images": true,
-    "files": false,
-    "voice": false,
+    "files": true,
+    "voice": true,
     "reactions": false,
     "threads": true,
     "typing": true
   },
   "metadata": {
     "serverName": "My WebSocket Server"
-  }
+  },
+  "timestamp": 1703275200000
 }
 ```
 
-**Server → Client (Message):**
+**Capabilities Request:**
+```json
+{
+  "type": "request_capabilities"
+}
+```
+
+**Message:**
 ```json
 {
   "type": "message",
@@ -146,12 +189,39 @@ const ws = new WebSocket('ws://localhost:8765/ws?key=YOUR_API_KEY');
 
 ws.on('message', (data) => {
   const msg = JSON.parse(data.toString());
-  if (msg.type === 'message') {
-    console.log('Bot:', msg.content.text);
+  
+  switch (msg.type) {
+    case 'handshake':
+      console.log('Server capabilities:', msg.capabilities);
+      break;
+      
+    case 'request_capabilities':
+      // Send client capabilities
+      ws.send(JSON.stringify({
+        type: 'capabilities_response',
+        capabilities: {
+          text: true,
+          images: true,
+          files: false,
+          voice: false,
+          reactions: false,
+          threads: true,
+          typing: true
+        },
+        metadata: {
+          clientName: 'My App',
+          version: '1.0.0'
+        }
+      }));
+      break;
+      
+    case 'message':
+      console.log('Bot:', msg.content.text);
+      break;
   }
 });
 
-// Send message
+// Send message (after capabilities_response)
 ws.send(JSON.stringify({
   type: 'message',
   from: 'user-123',
@@ -167,7 +237,30 @@ import json
 
 def on_message(ws, message):
     data = json.loads(message)
-    if data['type'] == 'message':
+    
+    if data['type'] == 'handshake':
+        print(f"Server capabilities: {data['capabilities']}")
+        
+    elif data['type'] == 'request_capabilities':
+        # Send client capabilities
+        ws.send(json.dumps({
+            "type": "capabilities_response",
+            "capabilities": {
+                "text": True,
+                "images": True,
+                "files": False,
+                "voice": False,
+                "reactions": False,
+                "threads": True,
+                "typing": True
+            },
+            "metadata": {
+                "clientName": "My Python App",
+                "version": "1.0.0"
+            }
+        }))
+        
+    elif data['type'] == 'message':
         print(f"Bot: {data['content']['text']}")
 
 ws = websocket.WebSocketApp(