harness: fix guard detection failure on macOS bash 3.2

Replace mapfile (bash 4.0+) with bash 3.2-compatible while-read loop.
macOS ships bash 3.2.57 (last GPLv2 version); mapfile silently failed,
leaving STAGED array empty, so no security checks ran (conflict markers,
secrets, private keys all missed). Guard self-test now passes 12/12.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.claude/scripts/harness-guard.sh

@@ -25,7 +25,10 @@ if [ "${SKIP_HARNESS_GUARD:-0}" = "1" ]; then
 fi
 
 ISSUES=0
-mapfile -t STAGED < <(git diff --cached --name-only --diff-filter=ACM 2>/dev/null)
+STAGED=()
+while IFS= read -r file; do
+    STAGED+=("$file")
+done < <(git diff --cached --name-only --diff-filter=ACM 2>/dev/null)
 
 for f in "${STAGED[@]}"; do
     [ -n "$f" ] || continue