test(issue-57): the derivable audit-log root is recorded, not forwarded

New: prefix()/CLI forward three keys for a sibling binding and four for an
explicit root; create records the sibling on the row, omits it from the
pane's --env and still pins it in the injected publish commands; the
issue's scene (a pane with WORKSPACE_ROOT and MAM_JOBS_DIR only) moves its
audit log with --registry-dir and leaves the control root alone;
worktree_prepare's pane --env carries no DELEGATE_JOB_LOGS_DIR.

The three existing four-key forwarding assertions (user-approved) compare
FORWARDED_ENV = CP_ENV[:3]; CP_ENV and the negative assertions are kept.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_issue21_worktree_prereqs.py

@@ -493,6 +493,8 @@ def test_t2_14_undeterminable_change_set_is_refused(wt_repo, monkeypatch, tmp_pa
 # ---------------------------------------------------------------------------
 
 CP_ENV = ("WORKSPACE_ROOT", "AGENT_SESSIONS_YAML", "MAM_JOBS_DIR", "DELEGATE_JOB_LOGS_DIR")
+# Issue #57: the derivable audit-log root is recorded on the row, not forwarded
+FORWARDED_ENV = CP_ENV[:3]
 
 
 def _binding(root: Path) -> dict:
@@ -576,7 +578,7 @@ def test_t3_1_create_persists_and_forwards_a_foreign_binding(mam_sandbox, mock_h
     (row,) = _rows(tmp)
     assert row["control_plane"] == _binding(tmp)
     flags = _env_flags(mock_herdr)
-    for k, v in zip(CP_ENV, _binding(tmp).values()):
+    for k, v in zip(FORWARDED_ENV, list(_binding(tmp).values())[:3]):
         assert f"{k}={v}" in flags, flags
     assert "WORKSPACE_ROOT=" not in row["pane"]["cmd_full"]
     assert "WORKSPACE_ROOT=" not in row["start_command"]
@@ -625,7 +627,7 @@ def test_t3_3_4_stop_keeps_the_binding_and_resume_reforwards_it(mam_sandbox, moc
     # tokens to the agent as arguments, review R1): resume takes a new pane,
     # whose creation call carries the binding as --env.
     flags = _env_flags(mock_herdr, since)
-    for k, v in zip(CP_ENV, _binding(tmp).values()):
+    for k, v in zip(FORWARDED_ENV, list(_binding(tmp).values())[:3]):
         assert f"{k}={v}" in flags, (res.stderr[-1500:], flags)
     _assert_no_env_in_agent_args(mock_herdr, since)
     (row,) = _rows(tmp)