fix(scripts): prune never removes prod skills silently

install.ps1/-sh -Prune: a prod skill with no source in skills/ is
potentially lost knowledge. Now:
- interactive: ask per skill (default No)
- non-interactive: skip with warning
- -Yes/--yes: remove, but warn loudly
Also fixes WinPS 5.1 parse break: removed em-dash (UTF-8 0x94 reads as
" in cp1251) from install.ps1 — file is ASCII-clean again.
Lesson: pi-add-models got pruned because it lived only in prod, no source.

scripts/install.ps1

@@ -1,19 +1,23 @@
 # Install skills/<name>/ into BOTH agent skill dirs:
-#   ~\.claude\skills\<name>\  (Claude Code — native, not configurable)
-#   ~\.agents\skills\<name>\  (pi — native default scan path, agent-neutral namespace)
+#   ~\.claude\skills\<name>\  (Claude Code - native, not configurable)
+#   ~\.agents\skills\<name>\  (pi - native default scan path, agent-neutral namespace)
 # $env:CLAUDE_SKILLS_DIR overrides ONLY the claude target (for testing/CI).
 #
-# Usage: install.ps1 [-Names <name1>,<name2>] [-Prune]
+# Usage: install.ps1 [-Names <name1>,<name2>] [-Prune] [-Yes]
 #   no args        = install all skills/* into both targets
 #   -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)
+#                    (prune always scans full target, ignores -Names filter - it's a global cleanup)
+#   -Yes           = with -Prune: remove prod-only skills without asking (still warns loudly).
+#                    Without -Yes, prune asks per skill (default No) and NEVER removes silently:
+#                    a prod skill with no source in skills/ is potentially lost knowledge.
 #
-# PowerShell port of install.sh — same behavior, native cmdlets, no bash dependency.
+# PowerShell port of install.sh - same behavior, native cmdlets, no bash dependency.
 
 [CmdletBinding()]
 param(
     [string[]]$Names = @(),
-    [switch]$Prune
+    [switch]$Prune,
+    [switch]$Yes
 )
 
 $ErrorActionPreference = 'Stop'
@@ -21,7 +25,7 @@ $ErrorActionPreference = 'Stop'
 $root = Split-Path -Parent $PSScriptRoot
 $src  = Join-Path $root 'skills'
 
-# Dual install targets — the "canon" is the git repo (skills/); both dirs are installs.
+# Dual install targets - the "canon" is the git repo (skills/); both dirs are installs.
 $targets = @()
 if ($env:CLAUDE_SKILLS_DIR) {
     $targets += $env:CLAUDE_SKILLS_DIR
@@ -62,12 +66,28 @@ foreach ($target in $targets) {
 
 if ($Prune) {
     $sourceNames = @(Get-ChildItem -Path $src -Directory | Select-Object -ExpandProperty Name)
+    $interactive = -not [Console]::IsInputRedirected
     foreach ($target in $targets) {
         $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
+                $rel = $dir.Name
+                $loc = $dir.FullName
+                if ($Yes) {
+                    Write-Warning "prune: $rel (not in skills/) - REMOVED from $loc"
+                    Remove-Item -Recurse -Force $loc
+                } elseif ($interactive) {
+                    Write-Warning "prune: $rel is in prod but has NO source in skills/ - possibly lost knowledge."
+                    $ans = Read-Host "  Remove '$rel'? [y/N]"
+                    if ($ans -eq 'y' -or $ans -eq 'Y') {
+                        Remove-Item -Recurse -Force $loc
+                        Write-Host "removed: $rel"
+                    } else {
+                        Write-Host "kept: $rel (remove manually, or add it to skills/)"
+                    }
+                } else {
+                    Write-Warning "prune skipped: $rel (prod skill without source; non-interactive - nothing removed)"
+                }
             }
         }
     }