read_file on a workspace FIFO/socket blocked until the exec timeout —
the existing device guard is name-based (/dev/*, /proc/*) and cannot
see an arbitrary special file. Add _special_file_kind(): one os.stat
on the resolved path, refusing FIFO/socket/char/block devices with a
plain note ('no read was attempted') instead of hanging. Host-visible
filesystems only; regular files, dirs, and missing paths unchanged.
Also adds evals/readtool/: an A/B harness that runs the real AIAgent
against hostile-file fixtures (huge lockfile, one-line bundle, FIFO,
NFD filenames, lying extensions) and measures accuracy, turns, tool
calls, and tokens. Measured for this guard (3 reps, file-only arm):
qwen3.8-max fifo task tokens 122k -> 26k (-79%), turns 9.3 -> 5.0;
opus-4.8 tokens 40k -> 23k; accuracy held 1.00 both arms.
