[DevOps]: Add pre-commit config mirroring CI checks

[shauryagangrade]

Problem

CI enforces ruff lint/format and mypy, but there's no local hook, so
contributors only learn about violations after pushing (each run ~1-2 min).

Proposed approach

• Add .pre-commit-config.yaml running ruff check, ruff format --check,
  and mypy gcode (local or mirror hooks), plus a dev-dependency entry for
  pre-commit.
• Document pre-commit install in CONTRIBUTING.

Where to look

• .github/workflows/ci.yml:30-38 (the checks to mirror)
• pyproject.toml:16-25 (dev extras)

Acceptance criteria

• pre-commit run --all-files passes on a clean tree.
• CONTRIBUTING mentions the one-line install.

Difficulty

Low — good first issue.