Use grep -F in dmesg filter to prevent interface name treated as regex

grep {iface} treats dots and other special chars as regex metacharacters.
Switch to grep -F -- {iface} for fixed-string matching and to prevent
a leading dash in the interface name from being parsed as a grep flag.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

diagnose.py

@@ -68,7 +68,7 @@ class DiagnosticsRunner:
             f' echo "=== ip_route ===";'
             f' ip route show dev {q} 2>/dev/null;'
             f' echo "=== dmesg ===";'
-            f' dmesg 2>/dev/null | grep {q} | tail -50;'
+            f' dmesg 2>/dev/null | grep -F -- {q} | tail -50;'
             f' echo "=== lldpctl ===";'
             f' lldpctl 2>/dev/null || echo "lldpd not running";'
             f' echo "=== end ==="'