Tool-result file writes hardened against symlink/hardlink tricks

Persisting large tool results to disk is now hardened against symlink and hardlink tricks. The write walks the destination path component by component, refusing to write through a symlink or non-directory, deletes any existing symlink at the exact target before writing, and afterward rejects the write if the final path turns out to be a symlink, a non-regular file, or has more than one hardlink.