Add ruff linting/formatting config and format codebase

[crtahlin]

Sub-issue of #114 (developer experience review).

Task

• Add [tool.ruff] config to pyproject.toml: target-version = "py39", line-length = 120
• Rules: E, F, I, UP, W (ignore E501)
• Per-file ignores: tests/** gets F401, F811
• Add ruff>=0.4.0 to testing optional dependencies
• Update requires-python from >=3.8 to >=3.9 (matches CI matrix)
• Run ruff check --fix . then ruff format . to apply consistent style

Why

No automated code style enforcement exists. Agent and human PRs may have inconsistent formatting with no way to catch it before review.

[miles-on-nightshift]

Triage: classification: enhancement, priority: priority-medium

ruff config establishes consistent code style. Should pair with #122 (CI lint job).

[miles-on-nightshift]

Triage: feature / priority: low

Ruff linting standardizes code style. Good hygiene, no urgency.

Labels to add: enhancement, priority-low