docs: add lint/format guidance to CONTRIBUTING.md to prevent ruff CI failures

[Copilot]

CI lint job failed because elesync/web.py was committed with uppercase Unicode escape sequences (\u270F, \u26A1, \u2B07, \u22EE) that ruff normalizes to lowercase. CONTRIBUTING.md had no mention of ruff or pre-commit setup, making this easy to hit silently.

What this changes

• Dev setup: adds pre-commit install to the setup snippet so the ruff-format hook activates automatically on every commit
• New "Linting and formatting" section: documents the two ruff commands to run before pushing (ruff check / ruff format) and explicitly calls out that ruff normalizes Unicode escape sequences to lowercase — the exact pattern that caused this failure
• Code style: adds a cross-reference to the new linting section

How to verify

pip install -e ".[dev]"
ruff check elesync/ tests/ scripts/
ruff format --check elesync/ tests/ scripts/
mypy elesync/

All should pass with no issues.

Checklist

• Tests added/updated and python -m unittest discover -s tests is green
• Docs/README updated if behaviour changed
• No new core runtime dependencies (or justified in the PR)