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>

services/webhook/matrix.go

@@ -265,6 +265,12 @@ func (m matrixConvertor) WorkflowJob(p *api.WorkflowJobPayload) (MatrixPayload,
 	return m.newPayload(text)
 }
 
+func (m matrixConvertor) WorkflowStep(p *api.WorkflowStepPayload) (MatrixPayload, error) {
+	text, _ := getWorkflowStepPayloadInfo(p, htmlLinkFormatter, true)
+
+	return m.newPayload(text)
+}
+
 var urlRegex = regexp.MustCompile(`<a [^>]*?href="([^">]*?)">(.*?)</a>`)
 
 func getMessageBody(htmlText string) string {