feat(api): add assigned_agent to Task model (MS22-DB-003, MS22-API-003) (#591)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/api/prisma/migrations/20260301211000_ms22_task_assigned_agent/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "tasks" ADD COLUMN "assigned_agent" TEXT;

apps/api/prisma/schema.prisma

@@ -379,6 +379,7 @@ model Task {
   creatorId   String       @map("creator_id") @db.Uuid
   projectId   String?      @map("project_id") @db.Uuid
   parentId    String?      @map("parent_id") @db.Uuid
+  assignedAgent String?    @map("assigned_agent")
   domainId    String?      @map("domain_id") @db.Uuid
   sortOrder   Int          @default(0) @map("sort_order")
   metadata    Json         @default("{}")