fix(export): filter dot-notation metrics from Prometheus textfile output

The shell exporter collects ~25 metrics using Zabbix dot-notation
(nftban.daemon.cpu_percent, nftban.server.load_1m). These are valid
for Zabbix trapper but invalid Prometheus metric names — dots violate
the [a-zA-Z_:][a-zA-Z0-9_:]* pattern.

The export_prometheus() awk filter already strips |STRING| lines but
did not filter dot-notation names. If Prometheus export is enabled,
these would leak into nftban.prom and cause node_exporter parse errors.

Currently a latent defect (NFTBAN_EXPORT_PROMETHEUS defaults to false),
but this closes the gap before anyone enables it.

Pipeline audit finding: P-8
Ref: V191_PIPELINE_INTEGRITY_AUDIT.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: itcmsgr

cli/lib/nftban/exporters/nftban_unified_exporter_export.sh

@@ -198,6 +198,10 @@ PROM_HEADER
         # Skip Zabbix string metrics (containing |STRING| prefix)
         if (index($2, "|STRING|") == 1) next
 
+        # Skip dot-notation metrics (Zabbix format, invalid Prometheus names)
+        # Valid Prometheus: [a-zA-Z_:][a-zA-Z0-9_:]*
+        if (index($1, ".") > 0) next
+
         if (NF >= 2) {
             # Handle metrics with labels
             if ($1 ~ /{.*}/) {