fix(bootstrap): path-shaped branches ancestry without isdir (#850 review #551)

resolve_canonical_repo_root fallback now uses commonpath only — no
string split and no os.path.isdir gate — so MCP project_root =
branches/<wt> still resolves to the repo root when the path is not yet
on disk (#274 / review #551 regression).

tests/test_author_mutation_worktree.py

@@ -86,6 +86,13 @@ class TestAssessAuthorMutationWorktree(unittest.TestCase):
         self.assertTrue(result["proven"])
         self.assertFalse(result["block"])
 
+    def test_path_shaped_branches_ancestry_without_isdir(self):
+        """Review #551: commonpath recovery must not require on-disk isdir."""
+        fake_wt = "/repo/Gitea-Tools/branches/issue-274"
+        root = amw.resolve_canonical_repo_root(fake_wt, fake_wt)
+        self.assertEqual(root, "/repo/Gitea-Tools")
+        self.assertNotIn('split("/branches/")', open(amw.__file__).read())
+
 
 class TestPreflightIntegration(unittest.TestCase):
     def test_verify_preflight_blocks_control_checkout_with_test_porcelain(self):