Add federation worker and canonical handoffs

internal/domain/domain.go

@@ -59,9 +59,13 @@ type Task struct {
 	Estimate         *Estimate  `json:"estimate,omitempty"`
 	State            TaskState  `json:"state"`
 	Lease            *Lease     `json:"lease,omitempty"`
-	Version          int        `json:"version"`
-	Title            string     `json:"title,omitempty"`
-	Description      string     `json:"description,omitempty"`
+	// HandoffRef survives the queued interval between TaskReleased and the
+	// next router-owned TaskLeased event; it is the only artifact the worker
+	// may use for local pickup validation.
+	HandoffRef  string `json:"handoff_ref,omitempty"`
+	Version     int    `json:"version"`
+	Title       string `json:"title,omitempty"`
+	Description string `json:"description,omitempty"`
 }
 
 type Event struct {