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>

legacy/README.md

@@ -0,0 +1,9 @@
+# legacy/
+
+Original `system_sync.sh` / `system_restore.sh` kept for reference and
+historical diffing. **Do not run these in production** — they have known
+issues (no `set -euo pipefail`, broken `MACOS = ".macos"` assignment with
+spaces, `​` zero-width character before the `ln` call, deprecated
+`spctl --master-disable`, hard-coded iCloud paths, no error handling).
+
+Use the top-level `ssr` CLI instead. See [../README.md](../README.md).