Give each task an assigned colour across the UI; lead links tasks

api/openapi.yaml

@@ -863,7 +863,7 @@ components:
 
     Task:
       type: object
-      required: [id, project_id, title, merge_title, branch, worktree_path, base_ref, port_base, status, created_by, created_at]
+      required: [id, project_id, title, merge_title, branch, worktree_path, base_ref, port_base, color, status, created_by, created_at]
       properties:
         id: { type: string }
         project_id: { type: string }
@@ -873,6 +873,7 @@ components:
         worktree_path: { type: string }
         base_ref: { type: string }
         port_base: { type: integer, description: "The first of the task's block of ports (port_base to port_base+9), passed to its agent and terminal as PORT." }
+        color: { type: integer, description: "Index into the UI's task palette, assigned at creation: a colour no active task holds, the one given out longest ago." }
         status:
           type: string
           enum: [active, merged, discarded]