repo-hygiene: destructive-guard PreToolUse hook inert on Windows — bash resolves to WSL relay, fail-open

[kyle-sexton]

Problem

Every Bash call during a /repo-hygiene:clean session on Windows 11 emits:

PreToolUse:Bash hook error
Failed with non-blocking status code: <3>WSL (…Relay) ERROR: CreateProcessCommon:818: execvpe(/bin/bash) failed: No such file or directory

The frontmatter hook invokes bash, which resolves to the WSL relay (System32\bash.exe) instead of Git Bash. The failure is non-blocking, so the destructive-command guard advertised by SKILL.md (blocks rm -rf, git clean -f*, git reset --hard, recursive Remove-Item; CLEAN_GUARD_ACK=1 acknowledgement protocol) silently enforces nothing. 48 occurrences in one session — also a per-call latency/noise tax.

Evidence

Session fee18cc2 (2026-07-22), 82-repo fleet sweep: guard never blocked anything, including bulk rm -rf applies.

Fix direction

• Resolve the interpreter explicitly (Git Bash path discovery, or sh) rather than bare bash.
• Consider fail-closed: a safety hook whose interpreter cannot launch should block, not warn — same class as disk-hygiene: guard hook fails to launch on skill-frontmatter hooks referencing ${CLAUDE_PLUGIN_DATA} — live fail-open on 0.4.6 #983 (disk-hygiene guard fail-open) and the convention: false-green checks — surfaces report success while the capability is dead or findings invisible (liveness-assertion umbrella) #532 false-green umbrella.