Migrate repository dependency installation paths from pip to uv

[Copilot]

This updates the repository’s dependency-management workflow to use uv instead of pip across local setup, CI bootstrap, and autograder provisioning. The goal is a single package-management path in developer and automation environments.

• Build/dev workflow (Makefile)

  • Replaced python -m venv + pip install flow with uv venv and uv pip.
  • Updated package-upgrade target to use uv pip --upgrade.

• GitHub Actions setup action

  • Added astral-sh/setup-uv to install and cache uv.
  • Replaced dependency install step from pip install -r requirements.txt to uv pip install --system -r requirements.txt.

• Autograder bootstrap

  • Replaced python3-pip installation and pip3 install with uv install + uv pip install --system.

• Editor environment defaults

  • Updated VS Code Python env settings to use uv as the default package manager.

setup:
	uv venv .venv
	uv pip install --python .venv/bin/python \
		-r requirements.txt \
		-r extras/autograder/source/requirements.txt