refactor: create shared script library, fix ISO image pinning, document planned splits

- S21: Create scripts/lib/common.sh with shared logging, SSH, health check, mem_limit functions
- S18: Source common.sh from deploy-to-target.sh, deploy-tailscale.sh, first-boot-containers.sh
- S16: Fix 2 hardcoded images in ISO build, add missing image variables
- S19: Document planned 7-module split of build-auto-installer-iso.sh
- S20: Document planned 8-module split of first-boot-containers.sh

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/deploy-tailscale.sh

@@ -27,6 +27,9 @@ TARGET_DIR="/home/archipelago/archy"
 # Source pinned image versions (single source of truth)
 [ -f "$SCRIPT_DIR/image-versions.sh" ] && . "$SCRIPT_DIR/image-versions.sh"
 
+# Source shared utility library
+[ -f "$SCRIPT_DIR/lib/common.sh" ] && . "$SCRIPT_DIR/lib/common.sh"
+
 SSH_KEY="${ARCHIPELAGO_SSH_KEY:-$HOME/.ssh/archipelago-deploy}"
 SSH_OPTS="-o StrictHostKeyChecking=no -o ServerAliveInterval=15 -o ServerAliveCountMax=4 -o ConnectTimeout=10 -i $SSH_KEY"
 BUILD_SOURCE="archipelago@${DEFAULT_PRIMARY:-192.168.1.228}"