feat(deployment): replace monster scripts with modular helper scripts (#87)

- Add install.sh orchestrator with upgrade support
- Add 6 helper scripts (<100 lines each) replacing 700-800 line monsters
- Add deployment/linux/furt.service systemd template
- Support both fresh install and upgrade modes
- Platform-aware detection (OpenBSD/FreeBSD vs Linux)
- Skip user/service creation in upgrade mode
- Preserve existing configuration during updates
- Remove merkwerk dependency from production install script

Helper scripts:
- scripts/setup-user.sh - Create system user (_furt/furt)
- scripts/setup-directories.sh - Create directory structure
- scripts/sync-files.sh - Copy source files to installation
- scripts/create-service.sh - Create system service from templates
- scripts/validate-config.sh - Validate furt.conf syntax
- scripts/health-check.sh - Basic health check functionality

Closes DAW/furt#87

scripts/setup-directories.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# scripts/setup-directories.sh - Create directory structure for furt
+
+set -e
+
+# Detect operating system for config directory
+if [ "$(uname)" = "OpenBSD" ] || [ "$(uname)" = "FreeBSD" ]; then
+    CONFIG_DIR="/usr/local/etc/furt"
+    USER="_furt"
+    GROUP="_furt"
+else
+    CONFIG_DIR="/etc/furt"
+    USER="furt"
+    GROUP="furt"
+fi
+
+# Create directories
+mkdir -p "$CONFIG_DIR"
+mkdir -p /usr/local/share/furt
+mkdir -p /var/log/furt
+
+# Set ownership for log directory (service user needs write access)
+chown "$USER:$GROUP" /var/log/furt
+
+echo "Created directories:"
+echo "  Config: $CONFIG_DIR"
+echo "  Share: /usr/local/share/furt"
+echo "  Logs: /var/log/furt (owned by $USER)"
+