Fix shebang env (#817)

* fix: use /usr/bin/env to PATH search sh (nixos compatiblity)

* fix: reconcile repo hooks with template /usr/bin/env change

examples/git-hooks/post-checkout

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env sh
 # bd-hooks-version: 0.26.0
 #
 # bd (beads) post-checkout hook

examples/git-hooks/post-merge

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env sh
 # bd-hooks-version: 0.26.0
 #
 # bd (beads) post-merge hook

examples/git-hooks/pre-commit

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env sh
 # bd-hooks-version: 0.29.0
 #
 # bd (beads) pre-commit hook

examples/git-hooks/pre-push

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env sh
 # bd-hooks-version: 0.22.2
 #
 # bd (beads) pre-push hook