fix: handle Windows read-only git pack files on devcontainer clone

shutil.rmtree fails on Windows when removing .git/objects/pack/*.idx
files because they're marked read-only. Add onerror handler to chmod
before unlink.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stranma

setup_project.py

@@ -508,7 +508,11 @@ def setup_devcontainer(root: Path, *, devcontainer: str, egress_firewall: bool)
             # Remove .git from the clone (we don't want a submodule)
             git_dir = dc_dir / ".git"
             if git_dir.exists():
-                shutil.rmtree(git_dir)
+                # Handle Windows read-only pack files
+                def _remove_readonly(func, path, _):
+                    os.chmod(path, 0o700)
+                    func(path)
+                shutil.rmtree(git_dir, onerror=_remove_readonly)
             actions.append("  Cloned trailofbits/claude-code-devcontainer into .devcontainer/")
         except subprocess.CalledProcessError as e:
             actions.append(f"  WARNING: Failed to clone Trail of Bits devcontainer: {e}")