Add beszel-agent template stack

Template version of beszel-agent with environment variable placeholders
for easy deployment to multiple servers. Customize per server:

- BESZEL_SSH_KEY: SSH public key from Beszel hub
- BESZEL_FILESYSTEM: Disk to monitor (default: /dev/sda1)
- BESZEL_LISTEN_PORT: Port for agent (default: 45876)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

stacks/beszel-template.compose.yml

@@ -0,0 +1,17 @@
+services:
+  beszel-agent:
+    image: henrygd/beszel-agent:latest
+    container_name: Beszel-Agent
+    network_mode: host
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+    environment:
+      # Port for Beszel agent to listen on
+      LISTEN: ${BESZEL_LISTEN_PORT:-45876}
+      # SSH public key from Beszel hub server
+      # Generate with: ssh-keygen -t ed25519
+      KEY: ${BESZEL_SSH_KEY}
+      # Filesystem to monitor (run 'df -h' to see options)
+      # Common values: /dev/sda1, /dev/nvme0n1p1, /dev/mapper/ubuntu--vg-ubuntu--lv
+      FILESYSTEM: ${BESZEL_FILESYSTEM:-/dev/sda1}
+    restart: on-failure:5