test(issue-17): every test that sources lib.sh does it under its own WORKSPACE_ROOT (review grok, codex)

d198dd5 fixed one helper; the same defect was in eleven more modules
(reviews grok and codex R1/R2): their shells inherited the process
environment, so each `source lib.sh` rewrote <WORKSPACE_ROOT>/.mam/shim/herdr
- the checkout's own when the variable is unset or names this checkout (which
pytest_configure keeps), another workspace's under --noconftest.

- conftest: own_workspace_root, an opt-in fixture setting WORKSPACE_ROOT to
  <tmp_path>/.mam-test-state/workspace. Not autouse: runner.py and
  mqtt_common read WORKSPACE_ROOT in-process, and some tests unset it on
  purpose. mam_sandbox and explicit envs run later and still win.
- pytestmark usefixtures("own_workspace_root") in test_version_consistency,
  test_b19_headless_reconcile_fixes, test_b8_send_keys_verification,
  test_issue6_fail_closed, test_loop_cli, test_issue5_phase3_loop,
  test_issue5_phase4_runtime, test_o3_scoped_guard, test_issue25_plan_reuse,
  test_orc_onboard (their shells inherit os.environ, many with no env at all).
- test_orc_onboard (R2): SCRUBBED_ENV is taken at import and no longer
  carries WORKSPACE_ROOT; every call builds _scrubbed_env(root) with the root
  of the call (ws / cwd / mam_sandbox, else this test's).
- test_workspace_scope (unittest) and test_issue21_worktree_prereqs._mutate
  pin the root they already create.

Tests (test_issue17_workspace_root_isolation.py): W1 copies the checkout and
runs one test of each of those modules there with WORKSPACE_ROOT naming the
copy - the copy gets no .mam; W2 imports test_orc_onboard under a sentinel
root, then calls it under another - the sentinel stays empty. At d198dd5 both
fail (the copy gets .mam/shim/herdr; the sentinel gets it).

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

tests/conftest.py

@@ -177,6 +177,21 @@ def _isolate_mam_state_dirs(_ambient_control_plane, tmp_path, monkeypatch):
     monkeypatch.setenv("MAM_ENV_FILE", str(state / ".mam.env.absent"))
 
 
+@pytest.fixture
+def own_workspace_root(tmp_path, monkeypatch):
+    """Issue #17: every `source lib.sh` rewrites $WORKSPACE_ROOT/.mam/shim/herdr,
+    and lib.sh derives this checkout when the variable is unset (a value naming
+    this checkout survives pytest_configure). A module whose shells inherit the
+    process environment opts in with `pytestmark = pytest.mark.usefixtures(
+    "own_workspace_root")`: they then write under this test's tmp_path instead.
+    mam_sandbox and a test's own WORKSPACE_ROOT run later and still win. Not
+    autouse: in-process code reads WORKSPACE_ROOT too (runner.py, mqtt_common)."""
+    root = tmp_path / ".mam-test-state" / "workspace"
+    root.mkdir(parents=True, exist_ok=True)
+    monkeypatch.setenv("WORKSPACE_ROOT", str(root))
+    return root
+
+
 @pytest.fixture
 def mam_sandbox(tmp_path, monkeypatch, request):
     """