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

telemetry/traces.rs

@@ -0,0 +1,55 @@
+//! OpenTelemetry distributed tracing — OTLP exporter + tracing-opentelemetry bridge.
+
+use opentelemetry::trace::TracerProvider as _;
+use opentelemetry_otlp::{Protocol, SpanExporter, WithExportConfig};
+use opentelemetry_sdk::propagation::TraceContextPropagator;
+use opentelemetry_sdk::trace::{SdkTracerProvider, Tracer};
+use opentelemetry_sdk::Resource;
+use tracing_opentelemetry::OpenTelemetryLayer;
+use tracing_subscriber::Registry;
+
+use super::config::{OtlpProtocol, TelemetryConfig};
+use crate::ImksResult;
+
+/// Build an OTLP SpanExporter based on the configured protocol.
+fn build_span_exporter(config: &TelemetryConfig) -> ImksResult<SpanExporter> {
+    match config.otlp_protocol {
+        OtlpProtocol::Grpc => SpanExporter::builder()
+            .with_tonic()
+            .with_endpoint(&config.otlp_endpoint)
+            .build()
+            .map_err(|e| crate::ImksError::Internal(format!("OTLP gRPC span exporter: {e}"))),
+        OtlpProtocol::HttpProtobuf => SpanExporter::builder()
+            .with_http()
+            .with_protocol(Protocol::HttpBinary)
+            .with_endpoint(&config.otlp_endpoint)
+            .build()
+            .map_err(|e| {
+                crate::ImksError::Internal(format!("OTLP HTTP span exporter: {e}"))
+            }),
+    }
+}
+
+/// Initialize the tracing pipeline: OTel tracer provider + tracing-opentelemetry layer.
+///
+/// Returns (SdkTracerProvider, OpenTelemetryLayer).
+pub fn init_tracing(
+    config: &TelemetryConfig,
+    resource: &Resource,
+) -> ImksResult<(SdkTracerProvider, OpenTelemetryLayer<Registry, Tracer>)> {
+    // Set global propagator for W3C TraceContext extraction/injection
+    opentelemetry::global::set_text_map_propagator(TraceContextPropagator::new());
+
+    let exporter = build_span_exporter(config)?;
+
+    let provider = SdkTracerProvider::builder()
+        .with_resource(resource.clone())
+        .with_batch_exporter(exporter)
+        .build();
+
+    let tracer = provider.tracer("imks");
+
+    let otel_layer = tracing_opentelemetry::layer().with_tracer(tracer);
+
+    Ok((provider, otel_layer))
+}