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?

Start by inventorying private/, dev/, and tools/private/, including the pyproject.toml, requirements, uv lock, and multirun target mentioned in the issue. Confirm the proposed layout with maintainers; done means dev-only tools and utilities are consolidated under dev and the ambiguous private locations are no longer needed.

Written by the indexing model from the issue text.