feat(telemetry): integrate OpenTelemetry observability stack with health metrics

- Add OpenTelemetry SDK, OTLP exporter, Prometheus integration
- Implement connection tracking with active/total/disconnection metrics
- Add health endpoint with uptime and connection counts
- Integrate tracing spans for socket events and engine messages
- Add metrics collection for event handling duration
- Update health endpoint to include live runtime state
- Add graceful telemetry shutdown in main function
- Implement engine session active metrics tracking
- Add namespace-specific attributes to connection metrics
- Introduce message edit history retrieval endpoint
- Add scheduled message CRUD operations and dispatcher
- Update Socket.IO event registration with observability
- Refactor component update to remove dead code allowance
- Add comprehensive environment variables documentation
- Implement detailed development guidelines in AGENTS.md

engine/session.rs

@@ -129,6 +129,12 @@ impl SessionStore {
                 sid
             );
         }
+        if let Some(m) = crate::telemetry::metrics::try_get() {
+            m.engine_sessions_active.add(
+                1,
+                &[opentelemetry::KeyValue::new("transport", transport.as_str())],
+            );
+        }
         rx
     }
 
@@ -137,7 +143,11 @@ impl SessionStore {
     }
 
     pub fn remove(&self, sid: &str) {
-        self.sessions.remove(sid);
+        if self.sessions.remove(sid).is_some()
+            && let Some(m) = crate::telemetry::metrics::try_get()
+        {
+            m.engine_sessions_active.add(-1, &[]);
+        }
     }
 
     pub fn exists(&self, sid: &str) -> bool {