fix(FN-5256): skip cycle preflight when new task has no deps

The unconditional listTasks() in assertNoDependencyCycle was wasted work
for the common no-dependency write and broke the same-agent duplicate
intake fail-open path: tests that stub listTasks to throw had the cycle
check consume the rejection before _maybeAutoArchiveSameAgentDuplicate's
try/catch could swallow it, propagating the error out of createTask.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/store.ts

@@ -3311,6 +3311,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     source: "createTask" | "createTaskWithReservedId" | "updateTask" | "replication",
     overrides?: Map<string, readonly string[]>,
   ): Promise<void> {
+    if (dependencies.length === 0 && !overrides) return;
     const lookup = await this.buildActiveTaskDependencyLookup(overrides);
     const cyclePath = detectDependencyCycle(taskId, dependencies, (candidateId) => lookup.get(candidateId));
     if (!cyclePath) return;