Implement XBPS packaging lifecycle for Void Linux (issue #85)

Add native XBPS package support with runit service lifecycle:
- packaging/xbps/install.sh: migration guard, fresh install (dormant),
  upgrade (snapshot, migrate, config preservation)
- packaging/xbps/remove.sh: sanctioned disable, purge (full cleanup)
- Makefile: package-xbps target with dependencies and config files
- tests/test_packaging.py: 5 XBPS-specific tests + updated shared tests

Acceptance criteria addressed:
- Fresh install remains dormant; runit down marker set
- Upgrade snapshots and migrates observation history safely
- Removal performs sanctioned pause and retains history
- Migration guard blocks install over make-install remnants
- Debian/RPM regressions verified via existing shared tests

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

packaging/xbps/remove.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+# XBPS REMOVE script — pre-remove and purge paths.
+#
+# Arguments: $1=ACTION $2=PKGNAME $3=VERSION $4=UPDATE $5=CONF_FILE $6=ARCH
+#
+# Actions: pre (before files removed), post (after files removed),
+#          purge (after metadata removed, for config cleanup)
+set -eu
+
+ACTION="$1"
+
+case "${ACTION}" in
+    pre)
+        # Sanctioned disable — close monitoring period
+        if [ -x /usr/libexec/fenris/fenris-monitor ]; then
+            /usr/libexec/fenris/fenris-monitor disable --now 2>/dev/null || true
+        fi
+        # runit: remove the service symlink and mark dormant
+        rm -f /var/service/fenris-collect
+        touch /etc/sv/fenris-collect/down 2>/dev/null || true
+        ;;
+    purge)
+        # Full cleanup — remove config, store, and runit service directories
+        rm -rf /etc/fenris
+        rm -rf /var/lib/fenris
+        if getent group fenris > /dev/null 2>&1; then
+            groupdel fenris 2>/dev/null || true
+        fi
+        rm -rf /etc/sv/fenris-collect 2>/dev/null || true
+        rm -rf /var/log/fenris-collect 2>/dev/null || true
+        ;;
+esac
+
+exit 0