[jhdark]

Closes #13.

Adds a lint job to CI that runs ruff check and ruff format --check.

Changes

• .github/workflows/ci.yml: new lint job using astral-sh/ruff-action@v3. It runs once rather than across the Python matrix, and reads the ruff version from pyproject.toml.
• pyproject.toml: new lint extra pinning ruff==0.17.3, and a [tool.ruff] section (src = ["src", "tests"] so first-party imports sort correctly).
• environment.yml: installs .[test,lint].
• README.md: dev commands now include ruff.
• docs/source/examples/linear_feedback.ipynb: sorted one import block, the only existing violation.

Choices on the open questions in #13

• Rules: ruff's default set. In 0.17 that's already broad (pyflakes, isort, pyupgrade, bugbear, RUF, SIM, …), so there's no select list for now. The version is pinned because the default set changes between ruff releases. Without a pin, local runs and CI could disagree.
• Formatting: enforced. The existing code is already ruff format-clean.

Locally, ruff check ., ruff format --check ., mypy src/pathsim_fusion and pytest (33 passed) all pass.