Implement idle/active mode for scalable agent connections

- Add StartStream/StopStream/AgentStatus messages to protobuf
- Agent now starts in idle mode (heartbeat only, no capture)
- Agent enters streaming mode when viewer connects (StartStream)
- Agent returns to idle when all viewers disconnect (StopStream)
- Server tracks viewer IDs and sends start/stop commands
- Heartbeat mechanism with 90 second timeout detection
- Session API now includes streaming status and agent info

This allows 2000+ agents to connect with minimal bandwidth.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

proto/guruconnect.proto

@@ -257,6 +257,27 @@ message Disconnect {
     string reason = 1;
 }
 
+// Server commands agent to start streaming video
+message StartStream {
+    string viewer_id = 1;           // ID of viewer requesting stream
+    int32 display_id = 2;           // Which display to stream (0 = primary)
+}
+
+// Server commands agent to stop streaming
+message StopStream {
+    string viewer_id = 1;           // Which viewer disconnected
+}
+
+// Agent reports its status periodically when idle
+message AgentStatus {
+    string hostname = 1;
+    string os_version = 2;
+    bool is_elevated = 3;
+    int64 uptime_secs = 4;
+    int32 display_count = 5;
+    bool is_streaming = 6;
+}
+
 // ============================================================================
 // Top-Level Message Wrapper
 // ============================================================================
@@ -293,6 +314,9 @@ message Message {
         Heartbeat heartbeat = 50;
         HeartbeatAck heartbeat_ack = 51;
         Disconnect disconnect = 52;
+        StartStream start_stream = 53;
+        StopStream stop_stream = 54;
+        AgentStatus agent_status = 55;
 
         // Chat
         ChatMessage chat_message = 60;