Add machine deletion API with uninstall command support

- Add AdminCommand message to protobuf (uninstall, restart, update)
- Add DELETE /api/machines/:agent_id endpoint with options:
  - ?uninstall=true - send uninstall command to online agent
  - ?export=true - return session history before deletion
- Add GET /api/machines/:agent_id/history endpoint for history export
- Add GET /api/machines endpoint to list all machines
- Handle AdminCommand in agent session handler
- Handle ADMIN_UNINSTALL error in agent main loop to trigger uninstall

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

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

agent/src/main.rs

@@ -416,7 +416,21 @@ async fn run_agent(config: config::Config) -> Result<()> {
                         return Ok(());
                     }
 
-                    error!("Session error: {}", e);
+                    if error_msg.contains("ADMIN_UNINSTALL") {
+                        info!("Uninstall command received from server - uninstalling");
+                        if let Err(e) = startup::uninstall() {
+                            warn!("Uninstall failed: {}", e);
+                        }
+                        show_message_box("GuruConnect Removed", "This computer has been removed from remote management.");
+                        return Ok(());
+                    }
+
+                    if error_msg.contains("ADMIN_RESTART") {
+                        info!("Restart command received - will reconnect");
+                        // Don't exit, just let the loop continue to reconnect
+                    } else {
+                        error!("Session error: {}", e);
+                    }
                 }
             }
             Err(e) => {