feat: T6.1 创作工具箱通用生成器框架地基 — 脑洞生成器 + run_generator

- schemas: Idea/IdeaListResult（脑洞生成器结构化产出，纯预览不入库）
- specs: brainstorm_spec（light 档，reads=[projects] writes=[]）
- orchestrator: build_brief_context（brief_only 策略）+ 通用 run_generator
  （据 spec.output_schema 校验 parsed，不绑定具体类型，任一声明式生成器共用）
- 新增 13 测；两包 ruff/format/mypy 干净、252 passed 无回归

P1 第一波（@llm）：证实「加生成器=加一份声明」。@backend T6.2/T6.3 待续。

packages/agents/ww_agents/__init__.py

@@ -15,6 +15,8 @@ from .schemas import (
     ForeshadowReview,
     ForeshadowSuggestion,
     ForeshadowWindow,
+    Idea,
+    IdeaListResult,
     OutlineChapter,
     OutlineResult,
     PaceIssue,
@@ -28,6 +30,7 @@ from .schemas import (
 )
 from .specs import (
     AgentSpec,
+    brainstorm_spec,
     character_gen_spec,
     continuity_spec,
     foreshadow_spec,
@@ -50,6 +53,8 @@ __all__ = [
     "ForeshadowReview",
     "ForeshadowSuggestion",
     "ForeshadowWindow",
+    "Idea",
+    "IdeaListResult",
     "OutlineChapter",
     "OutlineResult",
     "PaceIssue",
@@ -60,6 +65,7 @@ __all__ = [
     "StyleFingerprintResult",
     "WorldEntityCard",
     "WorldGenResult",
+    "brainstorm_spec",
     "character_gen_spec",
     "continuity_spec",
     "foreshadow_spec",