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>

agent/src/main.rs

@@ -204,8 +204,9 @@ fn cleanup_on_exit() {
 }
 
 async fn run_agent(config: config::Config) -> Result<()> {
-    // Create session manager
-    let mut session = session::SessionManager::new(config.clone());
+    // Create session manager with elevation status
+    let elevated = is_elevated();
+    let mut session = session::SessionManager::new(config.clone(), elevated);
     let is_support_session = config.support_code.is_some();
     let hostname = config.hostname();