test(issue-21): rec 3 - T2-6 isolates MAM_LOOP_WORKTREE/MAM_LOOP_BASE with monkeypatch

T2-6 set both variables in os.environ directly and restored them in a
finally block; a failing assertion between the two could still leak them to
later tests in the process. They are now set with monkeypatch.setenv (the
wt_repo fixture already delenvs both). The assertions are unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R3Eg7MEpSWnm1oB9ZPP6G

tests/test_issue21_worktree_prereqs.py

@@ -362,13 +362,12 @@ def test_t2_5b_prompts_unchanged_without_options(wt_repo, monkeypatch, tmp_path)
     assert _roles(calls, "Reviewer")[0].startswith("다음 구현 사항(작업 목표: t21)")
 
 
-def test_t2_6_worktree_env_fallback_and_no_base_env(wt_repo, tmp_path):
+def test_t2_6_worktree_env_fallback_and_no_base_env(wt_repo, tmp_path, monkeypatch):
     main, wt = wt_repo
     wt2 = tmp_path / "wt2"
     _git(main, "worktree", "add", "-q", "-b", "feat2", str(wt2))
-    os.environ["MAM_LOOP_WORKTREE"] = str(wt)
-    os.environ["MAM_LOOP_BASE"] = _git(main, "rev-parse", "HEAD")
-    try:
+    monkeypatch.setenv("MAM_LOOP_WORKTREE", str(wt))
+    monkeypatch.setenv("MAM_LOOP_BASE", _git(main, "rev-parse", "HEAD"))
     r = _runner()
     r.parse_args()
     assert r.worktree_root == Path(os.path.realpath(wt)) and r.worktree_given
@@ -376,9 +375,6 @@ def test_t2_6_worktree_env_fallback_and_no_base_env(wt_repo, tmp_path):
     r = _runner("--worktree", str(wt2))
     r.parse_args()
     assert r.worktree_root == Path(os.path.realpath(wt2)), "the CLI wins over MAM_LOOP_WORKTREE"
-    finally:
-        os.environ.pop("MAM_LOOP_WORKTREE", None)
-        os.environ.pop("MAM_LOOP_BASE", None)
 
 
 def test_t2_7_runner_reads_mam_jobs_dir(wt_repo, tmp_path, monkeypatch):