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-user.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# scripts/setup-user.sh - Create _furt system user and group
+
+set -e
+
+# Detect operating system
+if [ "$(uname)" = "OpenBSD" ] || [ "$(uname)" = "FreeBSD" ]; then
+    # BSD systems use _furt user convention
+    groupadd _furt 2>/dev/null || true
+    useradd -g _furt -s /bin/false -d /var/empty _furt 2>/dev/null || true
+    echo "Created BSD system user: _furt"
+else
+    # Linux systems use furt user with --system flag
+    groupadd --system furt 2>/dev/null || true
+    useradd --system -g furt -s /bin/false -d /var/empty furt 2>/dev/null || true
+    echo "Created Linux system user: furt"
+fi
+
+echo "User setup completed successfully"
+