test(issue-17): item-4 helpers run lib.sh in the test's own WORKSPACE_ROOT

_env kept an inherited WORKSPACE_ROOT, so every helper that sources the
wrapper (lib.sh's _init_herdr_isolation) rewrote <that workspace>/.mam/shim/
herdr - another workspace's during the PR #51 review (v4.8.7 review, claude).
Only dropping it is not enough: lib.sh then derives the checkout itself, and
conftest keeps a WORKSPACE_ROOT that names this checkout, so the run would
rewrite the checkout's own shim (the real one when this is the main
checkout). _env now sets WORKSPACE_ROOT to the test's tmp dir.

Test: with a sentinel WORKSPACE_ROOT in the environment, _run_both and a
direct _env run leave the sentinel empty and the checkout's shim untouched,
and the shell sees the tmp dir. With the old _env the sentinel gets
.mam/shim/herdr.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

tests/test_issue17_item4_decisions.py

@@ -222,10 +222,13 @@ def _shim(tmp_path: Path) -> Path:
 
 
 def _env(tmp_path: Path, **extra) -> dict:
+    # WORKSPACE_ROOT is this test's tmp_path, never inherited: sourcing the
+    # wrapper sources lib.sh, which rewrites $WORKSPACE_ROOT/.mam/shim/herdr -
+    # and unset, lib.sh would derive the checkout itself (Issue #17 follow-up).
     env = {k: v for k, v in os.environ.items()
-           if not k.startswith(("HERDR_", "MAM_", "SKS_")) and k not in ("PYTHONPATH",)}
+           if not k.startswith(("HERDR_", "MAM_", "SKS_")) and k not in ("PYTHONPATH", "WORKSPACE_ROOT")}
     env.update(MAM_DELEGATE_JOB_NO_MAIN="1", MAM_ENV_FILE=str(tmp_path / ".mam.env.absent"), HOME=str(tmp_path),
-               DELEGATE_JOB_LOGS_DIR=str(tmp_path / "logs"))
+               WORKSPACE_ROOT=str(tmp_path), DELEGATE_JOB_LOGS_DIR=str(tmp_path / "logs"))
     env.update({k: str(v) for k, v in extra.items() if v is not None})
     return env
 
@@ -464,6 +467,37 @@ def test_c_mam_int_and_knob(tmp_path, locale, minimum):
     assert outs[0] == outs[1], outs
 
 
+def _mtime(path: Path):
+    try:
+        return path.stat().st_mtime_ns
+    except OSError:
+        return None
+
+
+def test_c_helpers_write_nothing_to_an_inherited_workspace(tmp_path, monkeypatch):
+    """v4.8.7 review (claude): _env kept an inherited WORKSPACE_ROOT, so every
+    helper that sources the wrapper (lib.sh) rewrote <that workspace>/.mam/
+    shim/herdr - another workspace's, during the PR #51 review. With a sentinel
+    workspace in the environment, nothing appears under it, the checkout's own
+    shim is not rewritten either, and the shell sees the test's tmp dir."""
+    sentinel = tmp_path / "sentinel-ws"
+    sentinel.mkdir()
+    monkeypatch.setenv("WORKSPACE_ROOT", str(sentinel))
+    checkout_shim = REPO_ROOT / ".mam" / "shim" / "herdr"
+    before = _mtime(checkout_shim)
+    both = _run_both(tmp_path / "both", 'echo "ws=$WORKSPACE_ROOT"\n', {})
+    work = tmp_path / "direct"
+    work.mkdir()
+    res = subprocess.run(["bash", "-c", f'source "{WRAPPER}"; echo "ws=$WORKSPACE_ROOT"'], capture_output=True,
+                         text=True, cwd=str(work), timeout=60, env=_env(work))
+    assert sorted(str(p.relative_to(sentinel)) for p in sentinel.rglob("*")) == [], "written into the sentinel"
+    assert _mtime(checkout_shim) == before, "the checkout's shim was rewritten"
+    for rc, out, err, _ in both:
+        assert rc == 0 and "ws=<W>" in out, (rc, out, err)
+    assert res.returncode == 0 and f"ws={work}" in res.stdout, (res.stdout, res.stderr)
+    assert (work / ".mam" / "shim" / "herdr").is_file(), "lib.sh wrote its shim into the test's own root"
+
+
 # ---------------------------------------------------------------------------
 # S - static guard
 # ---------------------------------------------------------------------------