🐛 fix(test): pin REPO_DIR in sync integration test env

run_sync copied os.environ but never set REPO_DIR. In CI, test.yml
exports REPO_DIR (the real checkout) via GITHUB_ENV, so the inherited
value made the sync operate on the real CI checkout instead of the temp
fixture, deleting dirs later tests need. Pin REPO_DIR to the passed repo
and add a regression test proving an inherited REPO_DIR does not leak.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/test_build_tsl_playbook.py

@@ -242,6 +242,42 @@ class BuildTslPlaybookTests(unittest.TestCase):
                 )
                 self.assertNotEqual(result.returncode, 0, msg=f"{path} leaked")
 
+    def test_sync_ignores_inherited_repo_dir_from_environment(self):
+        if shutil.which("bash") is None:
+            self.skipTest("bash is required to run sync workflow script")
+
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            tmp = Path(tmp_dir)
+            repo = create_source_repo(tmp)
+
+            # Simulate the CI environment exporting REPO_DIR pointing at the
+            # real checkout. The sync must operate on the repo passed to
+            # run_sync, never the inherited value.
+            decoy = tmp / "decoy-checkout"
+            decoy.mkdir()
+            sentinel = decoy / "sentinel.txt"
+            sentinel.write_text("untouched\n", encoding="utf-8", newline="\n")
+
+            previous = os.environ.get("REPO_DIR")
+            os.environ["REPO_DIR"] = str(decoy)
+            try:
+                run_sync(repo)
+            finally:
+                if previous is None:
+                    os.environ.pop("REPO_DIR", None)
+                else:
+                    os.environ["REPO_DIR"] = previous
+
+            # The decoy must be left completely alone.
+            self.assertEqual(
+                sorted(item.name for item in decoy.iterdir()),
+                ["sentinel.txt"],
+            )
+            self.assertEqual(sentinel.read_text(encoding="utf-8"), "untouched\n")
+
+            # And the intended repo must actually have been published.
+            git(repo, "cat-file", "-e", "HEAD:AGENTS.md")
+
 
 def create_source_repo(tmp: Path) -> Path:
     repo = tmp / "repo"
@@ -265,6 +301,7 @@ def run_sync(repo: Path) -> None:
     env = os.environ.copy()
     env.update(
         {
+            "REPO_DIR": str(repo),
             "TARGET_BRANCH": "tsl-playbook",
             "COMMIT_AUTHOR_NAME": "test",
             "COMMIT_AUTHOR_EMAIL": "test@example.invalid",