feat(service): implement PID-file based service management

- Add PID directory creation in setup-directories.sh
- Update start.sh to use /var/run/furt/furt.pid for both platforms
- Fix OpenBSD rc.d script pidfile variable path
- Correct systemd service PIDFile parameter path
- Resolve rcctl check detection issues on OpenBSD

Fixes service detection problems where rcctl check would show (failed)
even when service was running. PID-file approach provides reliable
cross-platform service status detection instead of fragile pexp patterns.
Related DAW/furt#100

scripts/setup-directories.sh

@@ -18,12 +18,15 @@ fi
 mkdir -p "$CONFIG_DIR"
 mkdir -p /usr/local/share/furt
 mkdir -p /var/log/furt
+mkdir -p /var/run/furt
 
 # Set ownership for log directory (service user needs write access)
 chown "$USER:$GROUP" /var/log/furt
+chown "$USER:$GROUP" /var/run/furt
 
 echo "Created directories:"
 echo "  Config: $CONFIG_DIR"
 echo "  Share: /usr/local/share/furt"
 echo "  Logs: /var/log/furt (owned by $USER)"
+echo "  PID: /var/run/furt (owned by $USER)"