chore: gitignore dev artifacts (.venv, .coverage, htmlcov, .memsearch)

pyproject.toml declares pytest-cov as a dev dependency and configures
[tool.coverage.run], but the generated .coverage database and htmlcov/
report directory are not gitignored.  Also add .venv/ (standard Python
virtualenv) and .memsearch/ (session memory) to keep the working tree
clean for contributors.

.gitignore

@@ -15,3 +15,7 @@ variants/open/references/research.md
 __pycache__/
 *.pyc
 mise.toml
+.memsearch/
+.venv/
+.coverage
+htmlcov/