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.ps1

@@ -1,11 +1,15 @@
 # Install skills/<name>/ into ~\.claude\skills\<name>\ (or $env:CLAUDE_SKILLS_DIR)
-# Usage: install.ps1 [-Names <name1>,<name2>]   (no args = all)
+# Usage: install.ps1 [-Names <name1>,<name2>] [-Prune]
+#   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 -Names filter — it's a global cleanup)
 #
 # PowerShell port of install.sh — same behavior, native cmdlets, no bash dependency.
 
 [CmdletBinding()]
 param(
-    [string[]]$Names = @()
+    [string[]]$Names = @(),
+    [switch]$Prune
 )
 
 $ErrorActionPreference = 'Stop'
@@ -44,3 +48,14 @@ foreach ($name in $Names) {
     Copy-Item -Recurse $srcDir $dstDir
     Write-Host "installed: $name -> $dstDir"
 }
+
+if ($Prune) {
+    $sourceNames = @(Get-ChildItem -Path $src -Directory | Select-Object -ExpandProperty Name)
+    $installedDirs = Get-ChildItem -Path $target -Directory -ErrorAction SilentlyContinue
+    foreach ($dir in $installedDirs) {
+        if ($sourceNames -notcontains $dir.Name) {
+            Write-Host "pruning: $($dir.Name) (not in skills/) -> $($dir.FullName)"
+            Remove-Item -Recurse -Force $dir.FullName
+        }
+    }
+}