feat: selective sub-path backup for volatile dot-dirs

Mackup with engine=file_system already uses symlinks for all backed-up
apps (/Users/osurke/.docker → <cloud>/Mackup/mackup/.docker). The copy PHASE
fails when the source dir contains sockets or dangling symlinks.

For dirs like .claude that have a volatile debug/ subdir, the clean fix
is to list only the stable sub-paths in the Mackup cfg rather than the
whole directory — debug/ is never touched.

_SSR_MACKUP_SELECTIVE: built-in overrides for .claude, .cursor, .codex,
.vscode, .factory — lists only config files (settings.json, CLAUDE.md,
skills, plugins, agents, …), skipping logs/, debug/, etc.

_ssr_mackup_resolve_paths(): resolves the [configuration_files] entries
for a given item using the selective table, falling back to the whole dir.

ssr mackup scan now shows '6 sub-path(s) of .claude/' in the plan
instead of '.claude/' to make the selective behaviour visible.

Configurable via SSR_MACKUP_SELECTIVE_PATHS in ssr.conf.

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

lib/mackup.sh

@@ -24,6 +24,23 @@ _SSR_MACKUP_CURATED=(
     bin              # personal ~/bin scripts
 )
 
+# Selective sub-path overrides.
+# When a directory contains volatile/problematic files (sockets, debug
+# symlinks), list only the STABLE sub-paths here instead of the whole dir.
+# Format: "dir=subpath1,subpath2,subpath3"  (paths relative to the dir)
+#
+# Example: .claude has a debug/ dir with dangling symlinks → list only the
+# config files we actually need.  Extend via SSR_MACKUP_SELECTIVE_PATHS
+# (same format, space-separated entries) in ssr.conf.
+_SSR_MACKUP_SELECTIVE=(
+    ".claude=settings.json,CLAUDE.md,skills,plugins,agents,commands"
+    ".cursor=settings.json,keybindings.json,snippets,rules"
+    ".codex=auth.json,config.toml"
+    ".vscode=settings.json,keybindings.json,snippets"
+    ".vscode-insiders=settings.json,keybindings.json,snippets"
+    ".factory=settings.json,droids"
+)
+
 # Dot-dirs/files to never auto-include.
 # Covers: caches, generated state, credentials, large build artefacts.
 _SSR_MACKUP_SKIP=(
@@ -52,17 +69,56 @@ _ssr_mackup_cfg_name() {
     printf 'ssr-%s' "$name"
 }
 
+# Resolve the list of [configuration_files] entries for an item.
+# If a selective override exists for the item, expand sub-paths that are
+# present on disk.  Falls back to the item itself (whole directory).
+#
+# Outputs one path per line.
+_ssr_mackup_resolve_paths() {
+    local item="$1"
+    local -a all_selective=("${_SSR_MACKUP_SELECTIVE[@]}")
+    # Merge user-defined selectives from ssr.conf.
+    local entry
+    for entry in ${SSR_MACKUP_SELECTIVE_PATHS:-}; do
+        all_selective+=("$entry")
+    done
+
+    for entry in "${all_selective[@]}"; do
+        local key="${entry%%=*}"
+        local val="${entry#*=}"
+        if [[ "$key" == "$item" ]]; then
+            # Split comma-separated sub-paths; only include those that exist.
+            local sub
+            local IFS=','
+            for sub in $val; do
+                IFS=' '
+                local full="$item/$sub"
+                # List the path even if it doesn't exist yet — it may be created later.
+                printf '%s\n' "$full"
+            done
+            return 0
+        fi
+    done
+
+    # No selective entry — back up the whole directory.
+    printf '%s\n' "$item"
+}
+
 # Write a single ~/.mackup/<name>.cfg for one dot-path.
-# Skips writing if the file already exists and its content matches
-# (idempotent on repeated syncs).
+# Uses selective sub-paths when a matching entry exists in _SSR_MACKUP_SELECTIVE.
+# Skips writing if content is unchanged (idempotent).
 _ssr_mackup_write_cfg() {
     local cfg_dir="$1" item="$2"
     local app_name; app_name="$(_ssr_mackup_cfg_name "$item")"
     local cfg_file="$cfg_dir/${app_name}.cfg"
 
+    # Build [configuration_files] block.
+    local paths_block
+    paths_block="$(_ssr_mackup_resolve_paths "$item")"
+
     local desired
     desired="$(printf '[application]\nname = %s\n\n[configuration_files]\n%s\n' \
-        "$app_name" "$item")"
+        "$app_name" "$paths_block")"
 
     if [[ -f "$cfg_file" ]] && [[ "$(cat "$cfg_file")" == "$desired" ]]; then
         return 0   # already up-to-date