Add pre-commit hooks for the quality gates

[alimaandev]

Problem

Contributors must remember the local quality gates manually; CI catches them later, which is slow feedback.

Task

Add a pre-commit config (.pre-commit-config.yaml) running ruff and a quick syntax check on commit, with a note in CONTRIBUTING.md on installing it (pre-commit install).

Acceptance criteria

• pre-commit run --all-files passes on the current tree
• Config is minimal (ruff + trailing-whitespace/end-of-file checks)
• CONTRIBUTING.md documents installation

Files

• .pre-commit-config.yaml (new)
• CONTRIBUTING.md