feat(gateway): add node capability negotiation foundation

docs/api/PROTOCOL.md

@@ -21,6 +21,7 @@ The gateway provides:
 - **JSON-RPC 2.0**: Structured request/response protocol
 - **Streaming Events**: Real-time updates during agent processing
 - **HTTP Server**: Serves static dashboard and handles webhook endpoints
+- **Node Capability Negotiation**: Optional companion-node role/capability registration
 
 ### Execution Model (Sessions + Per-Session Queue)
 
@@ -563,6 +564,50 @@ When queue policy rejects/supersedes a request before execution, the server emit
 
 Cancel the current agent operation.
 
+### Node Methods
+
+#### `node.register`
+
+Register node role/capabilities for the current WebSocket connection.
+
+**Request:**
+```json
+{
+  "id": 9,
+  "method": "node.register",
+  "params": {
+    "nodeId": "companion-desktop",
+    "role": "companion",
+    "protocolVersion": 1,
+    "capabilities": ["ui.canvas", "notifications"]
+  }
+}
+```
+
+**Response:**
+```json
+{
+  "id": 9,
+  "result": {
+    "registered": true,
+    "node": { "id": "companion-desktop", "role": "companion" },
+    "protocol": { "serverVersion": 1, "clientVersion": 1, "negotiatedVersion": 1 },
+    "capabilities": {
+      "declared": ["ui.canvas", "notifications"],
+      "enabled": ["ui.canvas", "notifications"]
+    }
+  }
+}
+```
+
+#### `node.capabilities.get`
+
+Return negotiated capabilities for the currently registered node connection.
+
+#### `system.capabilities`
+
+Return gateway protocol version, node policy status, and feature-gate snapshot.
+
 **Request:**
 ```json
 {