Add Debian .deb package support

- debian/control, changelog, conffiles — package metadata
- debian/postinst — creates irc-bot system user, installs Python venv,
  symlinks runtime dirs, enables systemd services
- debian/prerm/postrm — clean stop/uninstall with purge support
- debian/systemd/ — hardened systemd units for bot and portal
- build_deb.sh — assembles and builds the .deb via dpkg-deb
- Path resolver in irc_client.py, memory.py, config_manager.py, portal/app.py:
  uses /etc/irc-bot + /var/lib/irc-bot when installed as .deb, relative
  paths otherwise (Docker/venv unchanged)

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

debian/prerm

@@ -0,0 +1,14 @@
+#!/bin/bash
+set -e
+
+case "$1" in
+  remove|upgrade|deconfigure)
+    if [ -d /run/systemd/system ]; then
+      systemctl stop irc-bot-portal.service 2>/dev/null || true
+      systemctl stop irc-bot.service 2>/dev/null || true
+      systemctl disable irc-bot.service irc-bot-portal.service 2>/dev/null || true
+    fi
+    ;;
+esac
+
+exit 0