A Zero-Padded Stdin Descriptor Crashes the Guard's int() Call #2028

_redirects_stdin in host-setup/agent-safety/claude/gh-write-guard.py calls int(fd) on a redirect's descriptor prefix after only a tok.isdecimal() check, so nothing bounds its length. Python 3.11 and later raise ValueError converting a digit string of more than 4300 digits, the hook exits 1, which is a non-blocking hook error, and the command runs with no check at all.

Bash reads a zero-padded descriptor as descriptor 0, so the crash is reachable with a command bash actually runs. A while read l; do sleep 30; done loop whose stdin redirect is spelled with 4400 zeros before <, reading /dev/zero, crashes the hook, where the same loop with a bare 0< is denied. A descriptor made of 4400 ones also crashes the hook, but bash rejects that line as a syntax error, so only the zero-padded form leaks.

The fix is the one feature/auto-1634 applies to the timeout signal number: strip leading zeros and compare the remainder, or bound its length, before any int() call. A self-test row with a zero-padded descriptor over 4300 digits pins it.

Found by a local strict review pass on feature/auto-1634 (#1634). The code predates that branch.