Stop tracking .coverage, and ignore local tool output

A `git add -A` in the previous commit swept up .coverage, the binary
SQLite file `pytest --cov` writes. It is machine-specific build output and
has no business in the tree.

Plain `pytest` - the command CONTRIBUTING.md documents - doesn't produce
it, which is why the gap went unnoticed: pytest and ruff both write
self-ignoring cache directories, and coverage.py does not. Ignore
.coverage, htmlcov and .ruff_cache so running any of the documented
commands leaves the tree clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MSNSYnceqvdVz7Csis4K9e

.gitignore

@@ -1,4 +1,7 @@
 __pycache__
 .vscode
 .venv
-.env
+.env
+.coverage
+htmlcov
+.ruff_cache