docs: add CLAUDE.md, move backup guide into installer, drop linux-to-sync

- CLAUDE.md: full contributor guide — service template, all helpers,
  globals, DRY_RUN convention, Caddy wiring, non-Docker patterns
- services/backup.sh: print backup strategy guide (Kopia/Borg/rsync/
  rsnapshot + when to use each) at the start of install_backup()
- README.md: remove standalone backup section, fix broken backup row,
  inline base package list, add CLAUDE.md to layout
- services/linux-to-sync.sh: deleted (never worked)
- setup.sh: remove linux-to-sync from is_installed()

https://claude.ai/code/session_019XgsQ13XKm4Zj3cNsDNwHj

setup.sh

@@ -80,7 +80,6 @@ is_installed() {
         glow) command -v glow >/dev/null 2>&1 ;;
         crowdsec) command -v cscli >/dev/null 2>&1 ;;
         silent-send) [ -d "$ACTUAL_HOME/silent-send/.git" ] ;;
-        linux-to-sync) [ -d "$ACTUAL_HOME/linux-to-sync/.git" ] ;;
         sync-cc) [ -f "$ACTUAL_HOME/sync-cc/sync_cc.py" ] ;;
         sky-cam) [ -d "$ACTUAL_HOME/sky-cam/.git" ] ;;
         *) [ -e "$DOCKER_DIR/$1" ] ;;