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>

units/runit/fenris-collect/log/run

@@ -0,0 +1,9 @@
+#!/bin/sh
+# fenris-collect runit log script — service output to /var/log/fenris-collect/
+#
+# Runit automatically pipes the service's stdout/stderr to this logger's stdin.
+# The logger writes to runit's log directory for diagnostics.
+#
+# Spec: §8.8 (actionable native diagnostics)
+exec chpst -u fenris:fenris \
+    svlogd -tt /var/log/fenris-collect/