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

services/backup.sh

@@ -25,6 +25,68 @@ register_service backup backup "Automatic encrypted backups (Kopia)"
 install_backup() {
     log_info "Setting up automatic encrypted backups (Kopia)..."
 
+    echo ""
+    echo "╔═══════════════════════════════════════════════════════════════════╗"
+    echo "║   BACKUP STRATEGIES — choose the right tool for your data        ║"
+    echo "╚═══════════════════════════════════════════════════════════════════╝"
+    echo ""
+    echo "  This installer sets up Kopia (the recommended default), but here"
+    echo "  is a quick guide to all available options so you can pick the"
+    echo "  right tool for each type of data."
+    echo ""
+    echo "  ┌─────────────────────────────────────────────────────────────────┐"
+    echo "  │ KOPIA (installed here) — block-level dedup + zstd + encryption  │"
+    echo "  │   Use for: files that change constantly — Minecraft worlds,     │"
+    echo "  │   game saves, Steam prefixes, Docker config volumes.            │"
+    echo "  │   Changed blocks are stored once; old blocks are shared.        │"
+    echo "  │   Restores via: kopia snapshot list / kopia restore             │"
+    echo "  └─────────────────────────────────────────────────────────────────┘"
+    echo ""
+    echo "  ┌─────────────────────────────────────────────────────────────────┐"
+    echo "  │ BORG (sudo apt install borgbackup) — chunk dedup + encryption   │"
+    echo "  │   Use for: same as Kopia. Choose Borg if you prefer Borgmatic   │"
+    echo "  │   (YAML config), Vorta (GUI), or multi-machine repos.           │"
+    echo "  │   borg init / borg create / borg list / borg extract            │"
+    echo "  └─────────────────────────────────────────────────────────────────┘"
+    echo ""
+    echo "  ┌─────────────────────────────────────────────────────────────────┐"
+    echo "  │ RSYNC plain  rsync -av --delete /src/ /dest/                    │"
+    echo "  │   Use for: media, ROMs, files that rarely change and you just   │"
+    echo "  │   need a copy. Fast, transparent — no special restore tool.     │"
+    echo "  │   Not suitable for files that change often (one bad --delete    │"
+    echo "  │   run immediately destroys the only copy in the destination).   │"
+    echo "  └─────────────────────────────────────────────────────────────────┘"
+    echo ""
+    echo "  ┌─────────────────────────────────────────────────────────────────┐"
+    echo "  │ RSYNC --link-dest  versioned snapshots, original folder layout  │"
+    echo "  │   Creates dated dirs (2024-01-15/, 2024-01-16/, …).            │"
+    echo "  │   Unchanged files are hard-linked — cost no extra disk space.  │"
+    echo "  │   Each dated dir is a complete, browsable snapshot of the       │"
+    echo "  │   source. Original folder structure preserved (unlike           │"
+    echo "  │   rsnapshot). If today's --delete wiped something, yesterday's  │"
+    echo "  │   dated dir is untouched.                                       │"
+    echo "  │   Use for: general files where you want versioning + readable   │"
+    echo "  │   snapshot dirs without a special restore tool.                 │"
+    echo "  └─────────────────────────────────────────────────────────────────┘"
+    echo ""
+    echo "  ┌─────────────────────────────────────────────────────────────────┐"
+    echo "  │ RSNAPSHOT (sudo apt install rsnapshot) — automated rotation     │"
+    echo "  │   Wraps rsync with a retention scheme (daily.0, weekly.0, …).  │"
+    echo "  │   Hard-links unchanged files like --link-dest, but dirs are     │"
+    echo "  │   named by rsnapshot (not your original structure).             │"
+    echo "  │   Use for: automated versioning without scripting --link-dest,  │"
+    echo "  │   when the rsnapshot naming convention doesn't bother you.      │"
+    echo "  └─────────────────────────────────────────────────────────────────┘"
+    echo ""
+    echo "  Quick reference:"
+    echo "    Constantly-changing data (saves, worlds, configs) → Kopia or Borg"
+    echo "    Media / ROMs (rarely changes, just need a copy)   → rsync plain"
+    echo "    Versioned snapshots, keep original folder layout  → rsync --link-dest"
+    echo "    Versioned snapshots, want auto rotation scripted  → rsnapshot"
+    echo ""
+    echo "  Continuing with Kopia setup..."
+    echo ""
+
     # ── Repo-conventional paths ──────────────────────────────────────────────
     local BACKUP_DIR="$DOCKER_DIR/backup"
     local CONF_FILE="$BACKUP_DIR/backup.conf"     # editable settings