Consolidate where dev-only tools and things live

[rickeylev]

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?

[aignas]

Dev sounds good