feat: make setup target for first-time contributors

[adamdaw]

Add a `make setup` target that installs all development dependencies and configures pre-commit hooks.

Context

First-time contributors must manually install pre-commit, npm tools, and Python tools following CONTRIBUTING.md. A single `make setup` command would streamline onboarding.

Acceptance Criteria

• Installs pre-commit hooks (`pre-commit install`)
• Installs Python dev tools (`pip install --constraint requirements-dev.txt pytest pytest-cov`)
• Verifies build dependencies (`make check`)
• Prints success message with next steps

Reference: CONTRIBUTING.md