Add watchdog-alert-check.sh (box watchdog-alerts helper script)

Self-contained FAILED-relaunch scanner for chromebox-watchdog.log
with watermark at NETVM_ROOT/watchdog-alert-watermark.txt.
Exits 0 quiet when clean, 1 with new lines printed when alerting.
The box-ctl.py watchdog-alerts action (in 2db0d92) drives this script.

Session: sidechat/chromebox-ops

bin/watchdog-alert-check.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+# watchdog-alert-check.sh - Check for new failed browser relaunches.
+#
+# Scans /home/super/Projects/NetVM/chromebox-watchdog.log for
+# "relaunch FAILED" lines newer than the watermark.
+#
+# Watermark: /home/super/Projects/NetVM/watchdog-alert-watermark.txt
+#   Holds the last reported log line (or is empty/missing on first run).
+#
+# Output contract:
+#   - No new failures: print nothing, exit 0.
+#   - New failures: print each new "relaunch FAILED" line, update the
+#     watermark to the newest line, exit 1.
+#
+# Self-contained: no arguments, no nested quoting. Safe to call from cron
+# or from the box CLI.
+
+set -u
+
+LOG="/home/super/Projects/NetVM/chromebox-watchdog.log"
+WATERMARK="/home/super/Projects/NetVM/watchdog-alert-watermark.txt"
+
+# No log file -> nothing to report.
+[ -f "$LOG" ] || exit 0
+
+# Gather all FAILED lines in chronological order.
+mapfile -t failed < <(grep "relaunch FAILED" "$LOG" 2>/dev/null || true)
+[ "${#failed[@]}" -gt 0 ] || exit 0
+
+# Read the watermark (last reported line). Missing/empty = report everything.
+watermark=""
+[ -f "$WATERMARK" ] && watermark="$(cat "$WATERMARK")"
+
+# Collect lines strictly newer than the watermark.
+new_lines=()
+if [ -z "$watermark" ]; then
+    new_lines=("${failed[@]}")
+else
+    seen_watermark=0
+    for line in "${failed[@]}"; do
+        if [ "$seen_watermark" -eq 1 ]; then
+            new_lines+=("$line")
+        elif [ "$line" = "$watermark" ]; then
+            seen_watermark=1
+        fi
+    done
+fi
+
+[ "${#new_lines[@]}" -gt 0 ] || exit 0
+
+# Report new failures and advance the watermark to the newest line.
+printf '%s\n' "${new_lines[@]}"
+printf '%s\n' "${failed[-1]}" > "$WATERMARK"
+exit 1