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/structs/hook.go

@@ -648,3 +648,26 @@ type WorkflowJobPayload struct {
 func (p *WorkflowJobPayload) JSONPayload() ([]byte, error) {
 	return json.MarshalIndent(p, "", "  ")
 }
+
+// WorkflowStepPayload represents a payload information of workflow step event.
+type WorkflowStepPayload struct {
+	// The action performed on the workflow step
+	Action string `json:"action"`
+	// The workflow run that contains the step
+	WorkflowRun *ActionWorkflowRun `json:"workflow_run"`
+	// The workflow job that contains the step
+	WorkflowJob *ActionWorkflowJob `json:"workflow_job"`
+	// The workflow step that was acted upon
+	Step *ActionWorkflowStep `json:"step"`
+	// The organization that owns the repository (if applicable)
+	Organization *Organization `json:"organization,omitempty"`
+	// The repository containing the workflow
+	Repo *Repository `json:"repository"`
+	// The user who triggered the workflow
+	Sender *User `json:"sender"`
+}
+
+// JSONPayload implements Payload
+func (p *WorkflowStepPayload) JSONPayload() ([]byte, error) {
+	return json.MarshalIndent(p, "", "  ")
+}