refactor: ssr-*.cfg files are user-owned after creation

ssr mackup scan now only suggests NEW files — it never overwrites an
existing ssr-*.cfg. Once created, the file belongs to the user who
edits it directly in ~/.mackup/ to customise which paths are backed up.

- _ssr_mackup_write_cfg: returns early if file already exists
- scan: drops UPD logic; only shows NEW suggestions
- _ssr_mackup_resolve_paths: removed SSR_MACKUP_SELECTIVE_PATHS support
  (ssr.conf config variable removed — edit the cfg files instead)
- Scan output: hints to edit files directly after creation

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

config/ssr.conf.example

@@ -26,11 +26,6 @@ SSR_MACKUP_AUTODISCOVER=true
 # Built-in defaults: .claude/debug .cursor/logs .vscode/logs .npm/_logs
 # SSR_MACKUP_PRECLEAN_PATHS=".my-app/tmp .other-app/run"
 
-# Selective sub-path overrides — back up only listed sub-paths for a dir
-# instead of the whole directory (avoids sockets/debug symlinks in volatile dirs).
-# Format: ".dir=subpath1,subpath2"  Built-in defaults cover .claude, .cursor, etc.
-# SSR_MACKUP_SELECTIVE_PATHS=".my-app=config.json,themes"
-
 # Extra paths (relative to $HOME) to always include, space-separated.
 # Example: ".easymcp .my-tool .config/my-cli"
 # SSR_MACKUP_EXTRA_PATHS=""