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).

author_mutation_worktree.py

@@ -93,9 +93,10 @@ def resolve_canonical_repo_root(workspace_path: str, fallback_project_root: str)
             pass
 
     # Fallback when git metadata is unavailable. Never string-split on
-    # "/branches/" (review #531 Finding 2 / F-6): recover the repo root only
-    # via resolved-path commonpath ancestry against a parent that owns a
-    # real ``branches`` directory containing the fallback path.
+    # "/branches/" (review #531 F2 / #551): recover the repo root only via
+    # resolved-path commonpath ancestry. Do **not** require on-disk isdir —
+    # MCP may launch with project_root = branches/<wt> before that path
+    # exists, and #274 path-shaped worktree-as-project-root must still resolve.
     fallback = os.path.realpath(fallback_project_root or workspace_path or ".")
     cur = fallback
     for _ in range(64):
@@ -103,16 +104,18 @@ def resolve_canonical_repo_root(workspace_path: str, fallback_project_root: str)
         if parent == cur:
             break
         branches_dir = os.path.realpath(os.path.join(parent, "branches"))
-        if os.path.isdir(branches_dir):
+        try:
+            # Path-shaped: fallback is under parent/branches/ (commonpath).
+            if os.path.commonpath([branches_dir, fallback]) == branches_dir:
+                return parent
+        except ValueError:
+            pass
+        # Fallback path itself is the branches directory.
+        if os.path.basename(os.path.realpath(cur)) == "branches":
             try:
-                if os.path.commonpath([branches_dir, fallback]) == branches_dir:
-                    return parent
-            except ValueError:
-                pass
-        # Also accept fallback itself being the branches directory.
-        if os.path.basename(cur) == "branches" and os.path.isdir(cur):
-            try:
-                if os.path.commonpath([cur, fallback]) == cur:
+                if os.path.commonpath([os.path.realpath(cur), fallback]) == os.path.realpath(
+                    cur
+                ):
                     return parent
             except ValueError:
                 pass