Add page-tree decision and Wagtail M2M implementation note

- 04-architecture 3.1: add ProjectIndexPage; authoritative page tree
  (project pages live under /projects/, never under scenario pages);
  ScenarioPage renders projects via query; filtering lives in
  ProjectIndexPage.get_context(), no separate Django view; Page M2M
  fields must use modelcluster ParentalManyToManyField
- 06-tasks: sync T-102 row/detail and T-204 quick-table accordingly
- routes: point project list page back to the 3.1 tree
- progress.md: append maintenance record

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/routes.md

@@ -35,6 +35,7 @@
 
 ### 项目列表页
 
+- 由 `ProjectIndexPage` 承载；页面树位置与查询方式见 [`04-architecture.md`](04-architecture.md) §3.1。
 - 展示所有已发布骨架项目。
 - 支持按语言、框架、数据库、AI 分数、关键词筛选。
 - 筛选条件体现在 URL 查询参数中；参数名、取值和组合规则以 [`04-architecture.md`](04-architecture.md) §3.4 契约表为唯一权威，本文不重复维护。