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>

pyproject.toml

@@ -33,13 +33,26 @@ documentation = "https://git-repos.zavage.net/Zavage-Software/app_skellington"
 
 [project.optional-dependencies]
 dev = [
-    "black",
+    "ruff==0.16.10",
     "pre-commit",
-    "isort",
-    "flake8"
 ]
 
 [tool.setuptools_scm]
 version_file = "app_skellington/_version.py"
 version_scheme = "release-branch-semver"
 local_scheme = "node-and-date"
+
+[tool.ruff]
+line-length = 88
+target-version = "py38"
+extend-exclude = ["app_skellington/_version.py"]
+
+[tool.ruff.lint]
+# E/W = pycodestyle, F = pyflakes, I = isort: the old flake8 + isort coverage.
+select = ["E", "F", "W", "I"]
+# The formatter owns line length. Long strings and comments it can't wrap are allowed.
+ignore = ["E501"]
+
+[tool.ruff.lint.per-file-ignores]
+# Placeholder smoke test. The characterization-tests plan rewrites it.
+"tests/test_cli.py" = ["F841", "E712"]