fix(hooks): use portable shebang for NixOS compatibility

.githooks/pre-push

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Block PRs by preventing pushes to arbitrary feature branches.
 # Gas Town agents push to main (crew) or polecat/* branches (polecats).
 # PRs are for external contributors only.