pr-reviews-test: resolve sleep and date from PATH, not /bin (improvise-3uow)

On the NixOS runners /bin holds only sh. With no /bin/sleep, the watchdog in
bounded_reviews exits at once and wins the `wait -n`, so every --wait case
reports 124. Both tools are now resolved with `type -P` before any case stubs
PATH.

Red: with both paths pointed at /nonexistent/, the suite fails 25 of 25.
Rival: REAL_SLEEP=/nonexistent/sleep fails 25 of 25 again. The date
pass-through has no discriminating rival, because pr-reviews only calls
`date +%s`. Evidence: context/test-evidence/tooling.md.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

scripts/pr-reviews-test.sh

@@ -9,12 +9,13 @@ work="$(mktemp -d)"
 trap 'rm -rf "$work"' EXIT
 
 # Every run is bounded, so a loop that never stops FAILS (exit 124) rather than
-# hanging the suite. /bin/sleep, because some cases put a no-op `sleep` first on PATH.
+# hanging the suite. Real tools resolved before any case stubs PATH; NixOS has no /bin/sleep.
 BOUND=20
+REAL_SLEEP="$(type -P sleep)"; export REAL_DATE; REAL_DATE="$(type -P date)"
 bounded_reviews() {
   "$here/pr-reviews" "$@" &
   local pid=$! dog done rc
-  /bin/sleep "$BOUND" &
+  "$REAL_SLEEP" "$BOUND" &
   dog=$!
   wait -n -p done "$pid" "$dog"; rc=$?
   if [ "$done" = "$pid" ]; then kill "$dog" 2>/dev/null; wait "$dog" 2>/dev/null; return "$rc"; fi
@@ -218,7 +219,7 @@ clock="$work/clock"
 mkdir -p "$clock"
 cat > "$clock/date" <<'EOF'
 #!/usr/bin/env bash
-[ "${1:-}" = "+%s" ] || exec /bin/date "$@"
+[ "${1:-}" = "+%s" ] || exec "$REAL_DATE" "$@"
 n=$(( $(cat "$CLOCK_TICKS" 2>/dev/null || echo 0) + 1 ))
 echo "$n" > "$CLOCK_TICKS"
 echo $(( 1000 + n ))