feat(webhook): add workflow_step webhook events for step-level CI notifications

Adds HookEventWorkflowStep event type that fires on every step state
transition (queued -> in_progress -> completed). Follows the same
pattern as the existing workflow_job events.

- New WorkflowStepPayload struct with run/job/step context
- WorkflowStepStatusUpdate notifier interface + dispatch
- Step state change detection in UpdateTask runner endpoint
- Fix: register workflow_step in updateHookEvents API mapping
- Full test coverage mirroring workflow_job tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/webhook/type.go

@@ -38,8 +38,9 @@ const (
 	HookEventPullRequestReview HookEventType = "pull_request_review"
 	// Actions event only
 	HookEventSchedule    HookEventType = "schedule"
-	HookEventWorkflowRun HookEventType = "workflow_run"
-	HookEventWorkflowJob HookEventType = "workflow_job"
+	HookEventWorkflowRun  HookEventType = "workflow_run"
+	HookEventWorkflowJob  HookEventType = "workflow_job"
+	HookEventWorkflowStep HookEventType = "workflow_step"
 )
 
 func AllEvents() []HookEventType {
@@ -70,6 +71,7 @@ func AllEvents() []HookEventType {
 		HookEventStatus,
 		HookEventWorkflowRun,
 		HookEventWorkflowJob,
+		HookEventWorkflowStep,
 	}
 }