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/rpm/post.sh

@@ -8,13 +8,33 @@ STORE_BAK="${STORE_DIR}/observations.db.bak"
 RUNTIME_PYTHON="/usr/bin/python3"
 VENDOR_DIR="/opt/fenris/vendor"
 
+# 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
+
 if [ "$1" -eq 1 ]; then
     # Fresh install
-    systemd-sysusers  || true
-    systemd-tmpfiles --create  || true
-    systemctl daemon-reload  || true
+    if [ "${INIT_SYSTEM}" = "systemd" ]; then
+        systemd-sysusers  || true
+        systemd-tmpfiles --create  || true
+        systemctl daemon-reload  || true
+    else
+        # runit: create group, set directory permissions
+        groupadd -f fenris
+        install -d -o root -g fenris -m 2750 "${STORE_DIR}" 2>/dev/null || true
+        # Mark runit service as dormant (down) for fresh install
+        if [ -d /etc/sv/fenris-collect ] && [ ! -e /var/service/fenris-collect ]; then
+            touch /etc/sv/fenris-collect/down
+        fi
+        # Ensure log directory exists
+        mkdir -p /var/log/fenris-collect
+        chown fenris:fenris /var/log/fenris-collect 2>/dev/null || true
+    fi
 elif [ "$1" -ge 2 ]; then
-    # Upgrade — snapshot, migration, daemon-reload, conditional timer restart
+    # Upgrade — snapshot, migration, init-system-aware reload
     if [ -f "${STORE_DB}" ]; then
         cp "${STORE_DB}" "${STORE_BAK}" 2>/dev/null || true
     fi
@@ -26,24 +46,26 @@ n = migrate_to_latest(Path('${STORE_DB}'))
 print(f'Fenris migration: {n} step(s) applied') if n else None
 " 2>&1 || echo "Fenris: migration skipped (store not yet initialized)"
     fi
-    # Capture running unit content BEFORE daemon-reload (spec §7)
-    RUNNING_UNITS=""
-    for unit in fenris-collect.timer; do
-        if systemctl is-active --quiet "${unit}" 2>/dev/null; then
-            RUNNING_UNITS="${RUNNING_UNITS} ${unit}"
-        fi
-    done
-    systemctl daemon-reload 2>/dev/null || true
-    # Restart timer only if unit contents changed AND active
-    for unit in ${RUNNING_UNITS}; do
-        OLD_CONTENT="$(mktemp)"
-        NEW_PATH="/usr/lib/systemd/system/${unit}"
-        systemctl cat "${unit}" > "${OLD_CONTENT}" 2>/dev/null || true
-        if ! diff -q "${OLD_CONTENT}" "${NEW_PATH}" > /dev/null 2>&1; then
-            systemctl restart "${unit}" 2>/dev/null || true
-        fi
-        rm -f "${OLD_CONTENT}"
-    done
-    # Re-apply placement modes (store dir group access, issue #54)
-    systemd-tmpfiles --create  || true
+    if [ "${INIT_SYSTEM}" = "systemd" ]; then
+        # Capture running unit content BEFORE daemon-reload (spec §7)
+        RUNNING_UNITS=""
+        for unit in fenris-collect.timer; do
+            if systemctl is-active --quiet "${unit}" 2>/dev/null; then
+                RUNNING_UNITS="${RUNNING_UNITS} ${unit}"
+            fi
+        done
+        systemctl daemon-reload 2>/dev/null || true
+        # Restart timer only if unit contents changed AND active
+        for unit in ${RUNNING_UNITS}; do
+            OLD_CONTENT="$(mktemp)"
+            NEW_PATH="/usr/lib/systemd/system/${unit}"
+            systemctl cat "${unit}" > "${OLD_CONTENT}" 2>/dev/null || true
+            if ! diff -q "${OLD_CONTENT}" "${NEW_PATH}" > /dev/null 2>&1; then
+                systemctl restart "${unit}" 2>/dev/null || true
+            fi
+            rm -f "${OLD_CONTENT}"
+        done
+        # Re-apply placement modes (store dir group access, issue #54)
+        systemd-tmpfiles --create  || true
+    fi
 fi