feat(install): add --prune flag to install.{ps1,sh} [skip-tdd: wrapper]

For each <name>/ dir in $target that has no matching skills/<name>/,
remove it. Catches stale installs after skill rename/retire
(motivating case: using-synology-ops just removed but install dir
lingered until manual rm).

Design choices:
- Combined flag (install + prune in one run), not standalone mode
- Prune always scans full target — does NOT respect -Names/positional
  filter, since stale-cleanup is a global concern
- Prints `pruning: <name>` per removal, no confirmation prompt
- Default off — flag must be passed explicitly

[skip-tdd: wrapper] — shell glue wrapping Remove-Item / rm -rf with
a set-difference; verified via smoke-test on disposable target dir
(fake stale dirs created, full install + prune run via env-overridden
CLAUDE_SKILLS_DIR, post-state confirmed: stale dirs removed, valid
skills installed, using-synology-ops absent as expected).

Closes 1/3 of [install-ps1] acceptance (the --prune flag). The
remaining doc .wiki/concepts/install-cross-platform.md is deferred
to a follow-up commit.

scripts/install.sh

@@ -1,13 +1,25 @@
 #!/usr/bin/env bash
 # Install skills/<name>/ into ~/.claude/skills/<name>/ (or $CLAUDE_SKILLS_DIR)
-# Usage: install.sh [name...]   (no args = all)
+# Usage: install.sh [--prune] [name...]
+#   no args        = install all skills/* into target
+#   --prune        = after install, remove target/<name>/ dirs that are NOT in skills/*
+#                    (prune always scans full target, ignores name filter — global cleanup)
 set -euo pipefail
 
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 SRC="$ROOT/skills"
 TARGET="${CLAUDE_SKILLS_DIR:-$HOME/.claude/skills}"
 
-if [ "$#" -eq 0 ]; then
+prune=0
+positional=()
+for arg in "$@"; do
+  case "$arg" in
+    --prune) prune=1 ;;
+    *) positional+=("$arg") ;;
+  esac
+done
+
+if [ "${#positional[@]}" -eq 0 ]; then
   # Portable across bash 3.2 (stock macOS) and bash 4+ (Linux, git-bash):
   # avoid `mapfile` (bash 4+) and `find -printf` (GNU find only).
   names=()
@@ -18,7 +30,7 @@ if [ "$#" -eq 0 ]; then
   IFS=$'\n' names=($(printf '%s\n' "${names[@]}" | sort))
   unset IFS
 else
-  names=("$@")
+  names=("${positional[@]}")
 fi
 
 mkdir -p "$TARGET"
@@ -38,3 +50,14 @@ for name in "${names[@]}"; do
   cp -R "$src_dir" "$dst_dir"
   echo "installed: $name → $dst_dir"
 done
+
+if [ "$prune" -eq 1 ]; then
+  for d in "$TARGET"/*/; do
+    [ -d "$d" ] || continue
+    name="$(basename "$d")"
+    if [ ! -d "$SRC/$name" ]; then
+      echo "pruning: $name (not in skills/) → $d"
+      rm -rf "$d"
+    fi
+  done
+fi