Initial import: nasmon health monitor, split out of home_nas

Go service (nasmon/) plus nfpm packaging (packaging/), carried over from
the home_nas monorepo. Adds rpm packaging alongside the existing deb
build (Fedora/RHEL/Rocky dependency names via nfpm overrides, per-format
systemd unit path, dpkg+rpm-compatible maintainer scripts) and a Gitea
Actions workflow that builds both formats on every push.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packaging/postrm.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+set -e
+if [ -d /run/systemd/system ]; then
+  systemctl daemon-reload || true
+fi
+if [ "$1" = "purge" ]; then
+  rm -rf /var/lib/nasmon /etc/nasmon
+fi
+exit 0