Add ruff + pre-commit (format + E,F,I)

[bordeauxred]

Why

No formatter or linter is currently enforced. Style will drift as external contributors open PRs. Add ruff + pre-commit with a narrow rule set that is safe to enable in one shot.

What

• Add [tool.ruff] config to pyproject.toml with line length 99, target py311, and E, F, I lint rules.
• Exclude benchmark/submodule paths from ruff.
• Add ruff>=0.4,<0.5 and pre-commit>=3.6 to the dev extra.
• Create .pre-commit-config.yaml with ruff and ruff-format hooks pinned to a known version.
• Run the one-shot format/lint pass in a separate commit for easy review.
• Add style instructions to CONTRIBUTING.md.

Acceptance

• ruff check --select E,F,I clawloop tests examples exits 0.
• ruff format --check clawloop tests examples exits 0.
• Pre-commit hook runs after pre-commit install.

Out of scope

• Broader semantic lint rule sets such as UP, B, and SIM.
• CI lint gating; that can be added separately after the first formatting pass lands.