Initial ssr repository

Convert the legacy system_sync.sh and system_restore.sh into a modular
shareable repository:

- bin/ssr CLI dispatcher with install/sync/restore/schedule/config/status
- lib/ split: common, cloud, brew, mackup, macos
- Cloud provider abstraction (icloud, dropbox, googledrive, onedrive, custom)
- launchd template for scheduled daily sync (renderable via sed)
- ssr.conf.example with whitelisted KEY=VALUE config loader
- macos-defaults.sh extracted from inline block, overridable
- install.sh bootstrap symlinks CLI into ~/.local/bin
- README with usage, config table, migration map, security notes
- Original scripts archived under legacy/ for reference

Fixes vs originals: set -euo pipefail, quoted vars, find -print0,
spctl --global-disable (was --master-disable), MACOS=".macos" bug,
zero-width char before ln, Apple Silicon brew shellenv, config injection
hardening.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/mackup.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# lib/mackup.sh — Mackup install + backup/restore.
+# Note: upstream mackup is in low-maintenance mode; we keep it because it
+# remains the simplest way to sync dotfiles for a wide range of apps. The
+# user can swap engines via ~/.mackup.cfg without changing this script.
+
+ssr::mackup::ensure_installed() {
+    if command -v mackup >/dev/null 2>&1; then
+        return 0
+    fi
+    ssr::log "Installing mackup via brew"
+    brew install mackup
+}
+
+# Writes ~/.mackup.cfg pointing the engine at the cloud directory.
+# Idempotent: rewrites only if content differs.
+ssr::mackup::configure() {
+    local mackup_dir="$1"
+    local cfg="$HOME/.mackup.cfg"
+    local desired
+    desired="$(printf '[storage]\nengine = file_system\npath = %s\ndirectory = mackup\n' "$mackup_dir")"
+
+    if [[ -f "$cfg" ]] && [[ "$(cat "$cfg")" == "$desired" ]]; then
+        return 0
+    fi
+
+    if [[ -L "$cfg" || -f "$cfg" ]]; then
+        cp -f "$cfg" "$cfg.bak.$(date +%s)" 2>/dev/null || true
+        rm -f "$cfg"
+    fi
+    printf '%s\n' "$desired" > "$cfg"
+    ssr::ok "Wrote $cfg"
+}
+
+ssr::mackup::backup() {
+    ssr::log "mackup backup"
+    mackup -f backup
+}
+
+ssr::mackup::restore() {
+    ssr::log "mackup restore"
+    mackup restore
+}