Consolidate where dev-only tools and things live

Currently, we have 3 places where dev-only tools and utilities live:

* private/ -- a multirun target to update requirements
* dev/ -- pyproject.toml, requirements, and uv lock
* tools/private/ -- various tools for dev-only purposes

I propose moving everything under dev. The rationale is that "private" is ambiguous -- do we mean "implementation detail" or "dev only" ?

@aignas thoughts?

先盘点 private/、dev/ 和 tools/private/，包括 issue 中提到的 pyproject.toml、requirements、uv lock 和 multirun target。与维护者确认提议的布局；完成意味着仅供 dev 使用的工具和实用程序已整合到 dev 下，并且不再需要含义不明确的 private 位置。

由索引模型根据 Issue 内容生成。