feat(03-01): hook metrics recording into agent request flow

- Track active requests with startRequest/endRequest around lane queue work
- Increment messagesProcessed on successful agent.process completion
- Record errors and error events on agent.send failures
- Record tool failure events with tool name and error details

src/gateway/handlers/agent.ts

@@ -37,6 +37,9 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
 
       // Enqueue the work — if the lane is idle it runs immediately,
       // otherwise it waits for earlier requests on the same session to finish.
+      const requestId = request.id.toString();
+      deps.metrics?.startRequest(requestId, { sessionId: laneId, channel: 'ws' });
+
       return deps.laneQueue.enqueue(laneId, async () => {
         deps.sessionBridge.setBusy(connectionId, true);
 
@@ -53,6 +56,17 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
                 error: event.result.error,
               } : undefined,
             }));
+            // Record tool failures as error events
+            if (event.result && !event.result.success) {
+              deps.metrics?.incrementErrors();
+              deps.metrics?.recordEvent({
+                timestamp: Date.now(),
+                level: 'error',
+                source: 'tool',
+                message: `Tool '${event.tool}' failed: ${event.result.error ?? 'unknown error'}`,
+                context: { sessionId: laneId, tool: event.tool },
+              });
+            }
           }
         });
 
@@ -66,13 +80,23 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
           }));
 
           const response = await agent.process(params.message!, attachments);
+          deps.metrics?.incrementMessages();
           send(makeEvent(request.id, 'done', { content: response }));
         } catch (err) {
           const message = err instanceof Error ? err.message : 'Unknown error';
+          deps.metrics?.incrementErrors();
+          deps.metrics?.recordEvent({
+            timestamp: Date.now(),
+            level: 'error',
+            source: 'agent.send',
+            message,
+            context: { sessionId: laneId },
+          });
           send(makeEvent(request.id, 'error', { code: ErrorCode.InternalError, message }));
         } finally {
           deps.sessionBridge.setBusy(connectionId, false);
           deps.sessionBridge.setOnToolUse(connectionId, undefined);
+          deps.metrics?.endRequest(requestId);
         }
       });
     },