infra: allow rerun make install without prompt

[kevinjqliu]

Rationale for this change

This PR lets uv manage the virtual env. uv will only setup a new venv if it does not exist and sync dep only when necessary.
This should make the entire make install process a lot faster and easier to work with

Context

I noticed running make install when a .venv already existed would show an interactive prompt:

➜  make install
uv is already installed.
uv venv 
Using CPython 3.12.11 interpreter at: /Users/kevinliu/.pyenv/versions/3.12.11/bin/python3
Creating virtual environment at: .venv
? A virtual environment already exists at `.venv`. Do you want to replace it? [y/n] › yes

hint: Use the `--clear` flag or set `UV_VENV_CLEAR=1` to skip this prompt

We dont need this prompt. And more crucially, claude keeps on getting stuck on this prompt 😞

Are these changes tested?

Yes running make install repeatedly

Are there any user-facing changes?

[rambleraptor]

This is a fantastic change! uv sync can be a very costly operation.

...especially if you're on a containerized environment where you might have a timeout to stand up a service (yes, I ran into a situation on a different project an hour ago)

[kevinjqliu]

since uv sync is super fast, its fine to call it (potentially) twice

[kevinjqliu]

@# is the syntax for comment. Using just # will have the sentence echo out when running make install

[geruh]

Makes sense can't really think of a way around this until we deprecate cython (if...)