fix(governance): validate completion pairs without a linked worktree

The canonical completion history validator materialised the completion tree with 'git worktree add', which writes administrative files into the reviewed repository's git directory. That directory is read-only inside the maintainer proof sandbox, so the maintainer preflight could not consult the registry that canonical governance already honours. Materialise the tree with a shared local clone instead: it reads the reviewed objects through alternates and writes only inside the disposable temporary directory, so CI and the maintainer pipeline validate the same reviewed pair without relaxing the sandbox boundary.

Change-source: pulse-maintainer

scripts/release_control/canonical_completion_history.py

@@ -87,29 +87,35 @@ def validate_completion(incomplete: str, head: str) -> bool:
     files = sorted(set(changed_files(incomplete) + changed_files(completion)))
     with tempfile.TemporaryDirectory(prefix="pulse-canonical-completion-") as temp:
         worktree = Path(temp) / "pulse"
-        git("worktree", "add", "--detach", str(worktree), completion)
-        try:
-            guard = worktree / "scripts/release_control/canonical_completion_guard.py"
-            result = subprocess.run(
-                [
-                    sys.executable,
-                    str(guard),
-                    "--files-from-stdin",
-                    "--diff-base",
-                    f"{incomplete}^",
-                    "--commit",
-                    completion,
-                ],
-                cwd=worktree,
-                input="".join(f"{path}\n" for path in files),
-                text=True,
+        # Materialise the completion tree with a shared clone rather than a
+        # linked worktree. A linked worktree writes its administrative files
+        # into the reviewed repository's git directory, which is read-only
+        # inside the maintainer proof sandbox. A shared clone reads the
+        # reviewed objects through alternates and writes only inside the
+        # disposable temporary directory, so CI and the maintainer preflight
+        # validate the pair identically without relaxing that boundary.
+        git("clone", "--quiet", "--shared", "--no-checkout", "--local",
+            str(REPO_ROOT), str(worktree))
+        git("-C", str(worktree), "checkout", "--quiet", "--detach", completion)
+        guard = worktree / "scripts/release_control/canonical_completion_guard.py"
+        result = subprocess.run(
+            [
+                sys.executable,
+                str(guard),
+                "--files-from-stdin",
+                "--diff-base",
+                f"{incomplete}^",
+                "--commit",
+                completion,
+            ],
+            cwd=worktree,
+            input="".join(f"{path}\n" for path in files),
+            text=True,
+        )
+        if result.returncode != 0:
+            raise ValueError(
+                f"registered completion {completion} does not complete {incomplete}"
             )
-            if result.returncode != 0:
-                raise ValueError(
-                    f"registered completion {completion} does not complete {incomplete}"
-                )
-        finally:
-            git("worktree", "remove", "--force", str(worktree))
 
     print(
         "Canonical completion history passed "