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/stage.sh

@@ -74,6 +74,12 @@ mkdir -p "${STAGE_DIR}/usr/lib/systemd/system"
 install -m 0644 "${REPO_ROOT}/units/fenris-collect.timer"   "${STAGE_DIR}/usr/lib/systemd/system/"
 install -m 0644 "${REPO_ROOT}/units/fenris-collect.service" "${STAGE_DIR}/usr/lib/systemd/system/"
 
+# --- runit service files ---
+echo "  Installing runit service files ..."
+mkdir -p "${STAGE_DIR}/etc/sv/fenris-collect/log"
+install -m 0755 "${REPO_ROOT}/units/runit/fenris-collect/run"     "${STAGE_DIR}/etc/sv/fenris-collect/run"
+install -m 0755 "${REPO_ROOT}/units/runit/fenris-collect/log/run" "${STAGE_DIR}/etc/sv/fenris-collect/log/run"
+
 # --- polkit policy ---
 echo "  Installing polkit policy ..."
 mkdir -p "${STAGE_DIR}/usr/share/polkit-1/actions"