fix: remove dangling symlinks before mackup backup

shutil.copy fails on dangling symlinks with [Errno 2] No such file or
directory (e.g. ~/.claude/debug/latest → missing target).

Pre-backup cleanup now handles two cases:
  - Unix sockets  (-type s)     → Errno 102
  - Dangling links (-type l !-e) → Errno 2

Both are ephemeral runtime artefacts that cannot be meaningfully backed up.

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

lib/mackup.sh

@@ -202,9 +202,10 @@ ssr::mackup::backup() {
     fi
     [[ -n "$mackup_root" ]] && ssr::mackup::_materialize_icloud "$mackup_root"
 
-    # Unix socket files (type s) cannot be copied and are always recreated
-    # by their owning app on startup.  Remove them from $HOME before backup
-    # to prevent shutil.Error: [Errno 102] Operation not supported on socket.
+    # Pre-backup cleanup: remove file types that shutil.copy cannot handle.
+    #
+    # 1. Unix socket files (-type s) — runtime artefacts, recreated on app start.
+    #    Cause: shutil.Error [Errno 102] Operation not supported on socket
     local socket_count
     socket_count="$(find "$HOME" -type s 2>/dev/null | wc -l | tr -d ' ')"
     if [[ "$socket_count" -gt 0 ]]; then
@@ -212,6 +213,15 @@ ssr::mackup::backup() {
         find "$HOME" -type s -delete 2>/dev/null || true
     fi
 
+    # 2. Dangling symlinks (-type l ! -e) — point to a non-existent target.
+    #    Cause: shutil.Error [Errno 2] No such file or directory: '<target>'
+    local dangling_count
+    dangling_count="$(find "$HOME" -type l ! -e 2>/dev/null | wc -l | tr -d ' ')"
+    if [[ "$dangling_count" -gt 0 ]]; then
+        ssr::log "Removing $dangling_count dangling symlink(s) before mackup backup"
+        find "$HOME" -type l ! -e -delete 2>/dev/null || true
+    fi
+
     local attempt rc=0
     for attempt in 1 2 3; do
         if mackup -f backup; then