style: adopt ruff (format + lint) at 88, replacing black/isort/flake8

Format-only: Python files are AST-identical apart from import order.
pre-commit end-of-file/trailing-whitespace fixes applied repo-wide.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -9,22 +9,20 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 ## Commands
 
 ```bash
-pip install -e '.[dev]' pytest   # dev extras = black, isort, flake8, pre-commit; pytest is NOT in the extras
+pip install -e '.[dev]' pytest   # dev extras = ruff, pre-commit; pytest is NOT in the extras
 pytest tests                     # full suite from repo root (or `pytest .` from inside tests/)
 pytest tests/cfg/test_cfg.py::TestConfig_e2e::test_uses_spec_as_defaults   # single test
 pytest tests -k "<keyword>"
 
-black app_skellington
-isort --profile black app_skellington
-flake8 app_skellington           # .flake8: max-line-length 120, tests/ excluded
+ruff check --fix                 # lint + import sorting (config in pyproject.toml)
+ruff format                      # formatter, line length 88
 
 python -m build                  # sdist + wheel into dist/
 python -m setuptools_scm         # print the version the build will get
 ```
 
 - **Versioning is tag-driven.** setuptools_scm uses `release-branch-semver` and derives the version from the latest `vX.Y.Z` git tag. It writes `app_skellington/_version.py` at build time. That file is tracked, but don't hand-edit it. `pip install -e .` also rewrites it, so `git restore` it rather than committing the churn. A release means tagging, then `python -m build`, then `twine upload dist/*`. The README covers the full publish steps.
-- **Formatter config is inconsistent.** `pyproject.toml` has no `[tool.black]`, so black runs at its default width of 88. Only flake8 allows 120. The code on `main` is formatted at 88. The pre-commit black hook pins `language_version: python3.8`. It fails unless a 3.8 interpreter is on PATH (`.python-version` names 3.8.x for pyenv).
-- **Lint is not currently clean on `main`.** black wants to reformat 2 files, isort flags 3, and flake8 reports W391 and W503. Keep unrelated reformatting out of feature diffs.
+- **ruff** handles formatting (88 columns) and lint (`E`, `F`, `W`, `I`). E501 is ignored on purpose, since the formatter owns line length. Keep the `ruff==` pin in `pyproject.toml` equal to the `rev:` in `.pre-commit-config.yaml`.
 - Tests cover only `cfg` (ConfigObj fixtures live in `tests/cfg/`) plus a CommandTree constructor smoke test. `tests/test_log.py` is empty.
 
 ## Python support policy