Graceful Handling of fchmod/fsync on Restricted Filesystems

The atomic file-write path now catches EINVAL, ENOTSUP, EPERM, and ENOSYS errors from fchmod and fsync calls and continues rather than aborting. This resolves hard write failures on certain network filesystems and Docker-mounted volumes that don't support those syscalls.

Previously, a fchmod error on a file that had just been written would leave the file in an inconsistent state. Now Claude Code logs the limitation (e.g. "fchmod unsupported on this filesystem") and continues normally.