feat(web): AI 立项方案生成+预填（种子门控 + 按字段接受）

向导阶段（project 未建）一键推演结构化书级蓝图预填立项向导。

- 新 SPEC project_plan_spec（analyst 档，纯预览 reads/writes=()）+ schema
  ProjectPlanResult（书名候选 + 时空背景/叙事结构/故事核心/结局设计/基调，
  全字段默认值守解析韧性）+ 注册 SCHEMA_CATALOG；计数三处 21→22 + regen golden。
- 端点 POST /skills/project-plan/generate（不带 project 前缀，绕开 toolbox 404）：
  请求体序列化向导草稿为 project_context，复用 build_brief_context + run_generator；
  种子门控（缺 genre/logline→422）；请求 schema 加 max_length 上界。
- run_generator/_build_request 的 project_id 放宽 uuid.UUID|None（向导无 project）。
- 前端 wizard.ts 加 mapPlanResultToWizardForm（错配字段折进 premise 不静默丢）+
  applyPlanPatch（仅回填空字段、保护已编辑）+ useProjectPlan hook + PlanAssistant。

apps/web/lib/wizard/useProjectPlan.ts

@@ -0,0 +1,59 @@
+"use client";
+
+import { useCallback, useState } from "react";
+
+import { useToast } from "@/components/Toast";
+import { api } from "@/lib/api/client";
+import { generationErrorMessage } from "@/lib/generation/cards";
+import type {
+  ProjectPlanGenerateRequest,
+  ProjectPlanView,
+} from "@/lib/api/types";
+
+export type ProjectPlanStatus = "idle" | "generating" | "done" | "error";
+
+export interface UseProjectPlan {
+  status: ProjectPlanStatus;
+  plan: ProjectPlanView | null;
+  generate: (seed: ProjectPlanGenerateRequest) => Promise<void>;
+  reset: () => void;
+}
+
+// AI 立项方案生成器：POST /skills/project-plan/generate → 结构化方案预览（不入库）。
+// 向导阶段调用（project 未建）；种子门控在后端亦校验（缺 genre/logline → 422）。
+export function useProjectPlan(): UseProjectPlan {
+  const [status, setStatus] = useState<ProjectPlanStatus>("idle");
+  const [plan, setPlan] = useState<ProjectPlanView | null>(null);
+  const toast = useToast();
+
+  const generate = useCallback<UseProjectPlan["generate"]>(
+    async (seed) => {
+      setStatus("generating");
+      setPlan(null);
+      try {
+        const { data, error } = await api.POST(
+          "/skills/project-plan/generate",
+          { body: seed },
+        );
+        if (error || !data) {
+          setStatus("error");
+          toast(generationErrorMessage(error), "error");
+          return;
+        }
+        setPlan(data);
+        setStatus("done");
+      } catch {
+        setStatus("error");
+        toast("生成请求异常，请检查网络。", "error");
+      }
+    },
+    [toast],
+  );
+
+  const reset = useCallback((): void => {
+    setStatus("idle");
+    setPlan(null);
+  }, []);
+
+  return { status, plan, generate, reset };
+}