Implement runit support for Fenris monitoring (issue #84)

Deliver end-to-end native monitoring path under runit with existing
CLI/TUI controls and truthful status, preserving systemd behavior.

Changes:
- Add init system abstraction layer (src/fenris/init_system.py) that
  detects systemd vs runit and provides unified interface for timer
  control, on-demand collection, and service state queries
- Create runit service files (units/runit/) with completion-relative
  5-minute cadence, 2-minute boot delay, bounded execution (90s),
  no catch-up, and serialized runs via flock
- Update monitor.py to use abstraction layer instead of direct systemctl
- Update status.py to use abstraction layer for service state queries
- Update all packaging scripts (deb, rpm) for init-system-aware setup
- Update Makefile to install runit service files alongside systemd units
- Add 46 tests for init system abstraction layer

Spec: ADR 0008, §8.4, §8.5, §8.6, §8.7, §8.8
Closes #84

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

packaging/postrm.sh

@@ -5,6 +5,13 @@
 #             postrm purge     (after conffiles and config removed)
 set -eu
 
+# Detect init system
+if [ -d /run/systemd/system ] || [ "$(cat /proc/1/comm 2>/dev/null)" = "systemd" ]; then
+    INIT_SYSTEM="systemd"
+else
+    INIT_SYSTEM="runit"
+fi
+
 case "${1:-}" in
     purge)
         rm -rf /etc/fenris
@@ -16,4 +23,10 @@ case "${1:-}" in
     remove|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
         ;;
 esac
-systemctl daemon-reload 2>/dev/null || true
+if [ "${INIT_SYSTEM}" = "systemd" ]; then
+    systemctl daemon-reload 2>/dev/null || true
+else
+    # runit: clean up service directory and log
+    rm -rf /etc/sv/fenris-collect 2>/dev/null || true
+    rm -rf /var/log/fenris-collect 2>/dev/null || true
+fi