feat: Replace run_checks.sh with prek pre-commit hooks

[corbt]

Summary

Replaces the custom run_checks.sh script with prek, a fast Rust-based pre-commit framework.

Changes

• Add prek as dev dependency for faster, single-binary pre-commit hooks
• Add .pre-commit-config.yaml with:
  • ruff + ruff-format (from astral-sh/ruff-pre-commit)
  • pyright type checking
  • uv lock --check for lockfile sync verification
  • pytest (manual stage locally, runs in CI)
• Rename workflow from ruff.yml to prek.yml, now runs all hooks via prek
• Delete run_checks.sh (superseded by prek hooks)
• Update docs (CONTRIBUTING.md, AGENT.md) with new workflow

Local Usage

uv sync --group dev
uv run prek install        # optional: install git hooks
uv run prek run --all-files
uv run prek run pytest     # run tests manually

Why prek?

• Single binary, no Python runtime needed for the hook runner
• Faster than pre-commit, shared toolchains between hooks
• Drop-in compatible with existing pre-commit configs
• Built-in uv integration for Python environments