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

svc/message.rs

@@ -296,6 +296,43 @@ impl MessageService {
         Ok(())
     }
 
+    /// Handle `message:edit_history` — retrieve the edit history for a message.
+    pub async fn get_edit_history(
+        &self,
+        socket: Arc<Socket>,
+        data: &serde_json::Value,
+    ) -> ImksResult<()> {
+        let user_id = self.user_id(&socket)?;
+        let payload = Self::first_payload(data)?;
+        let message_id: Uuid = Self::parse_field(payload, "message_id")?;
+
+        let message = self
+            .repo
+            .get(message_id)
+            .await?
+            .ok_or_else(|| ImksError::NotFound(format!("message {message_id}")))?;
+
+        let channel_id_str = message.channel_id.to_string();
+        let user_id_str = user_id.to_string();
+
+        self.ensure_readable(&channel_id_str, &user_id_str).await?;
+
+        let history = self.repo.get_edit_history(message_id).await?;
+        let summary = self.repo.get_edit_summary(message_id).await?;
+
+        let _ = socket.emit(
+            "message:edit_history",
+            serde_json::json!({
+                "message_id": message_id.to_string(),
+                "edits": history,
+                "edit_count": summary.edit_count,
+                "last_edited_at": summary.last_edited_at,
+                "last_edited_by": summary.last_edited_by,
+            }),
+        );
+        Ok(())
+    }
+
     // Permission validation helpers
 
     /// Full write-access gate: resolve channel + readability + membership + SEND_MESSAGE.
@@ -481,7 +518,7 @@ impl MessageService {
         }
     }
 
-    fn validate_body_size(&self, body: &str) -> ImksResult<()> {
+    pub(crate) fn validate_body_size(&self, body: &str) -> ImksResult<()> {
         if body.len() > self.max_body_size {
             return Err(ImksError::InvalidInput(format!(
                 "Message body exceeds max size of {} bytes (got {})",