chore: untrack committed developer virtualenv (build-env/) (#132)

Closes #106.

49 entries under `build-env/` were tracked in version control: pip/pytest
launchers with a hardcoded absolute interpreter path, the four shell
activation scripts, `pyvenv.cfg`, and a `lib64` symlink. These are
machine-specific environment artifacts, not SDK source.

- `git rm -r --cached build-env` removes them from the index only; a
  developer's actual on-disk environment is untouched.
- `.gitignore` gains `build-env/` plus `pyvenv.cfg` so a recreated
  environment is not re-added.
- `tests/test_no_tracked_virtualenv.py` asserts via `git ls-files` that no
  virtualenv directory or marker file is tracked, and that `build-env/` is
  ignored. All three assertions fail on the parent commit.

No history rewrite. No source, packaging or dependency declaration changed.
Nothing in the repository referenced `build-env` (verified by `git grep`
over all tracked files and a recursive grep over the working tree), and
neither the sdist nor the wheel ever contained these files.


Claude-Session: https://claude.ai/code/session_015ao5paex73xXvuM424Libo

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: karlwaldman

.gitignore

@@ -111,6 +111,9 @@ venv/
 ENV/
 env.bak/
 venv.bak/
+build-env/
+*/pyvenv.cfg
+pyvenv.cfg
 
 # Spyder project settings
 .spyderproject