Add pre-commit hooks (ruff)

[askmy-stack]

Background

Identified during OSS audit v2 of Py-Outlier.

Problem Statement

No pre-commit.

Proposed Solution

.pre-commit-config.yaml.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

.pre-commit-config.yaml

---

Suggested labels: priority:medium, good first issue, category:refactor
Milestone: v1.0-oss-ready
OSS Audit: Master Backlog v2