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

Symmetric to install-side --prune (e871c20). Removes
dist/<name>.skill files whose <name> is not in skills/*.

Design choices match install:
- Combined flag (build + prune in one run)
- Global scan, ignores -Names / positional filter
- Default off, print-and-delete, no confirmation

Bash wrinkle: when build.sh delegates to powershell.exe -File
build.ps1 (Windows-without-zip case), --prune is NOT forwarded.
Bash runs prune itself at the end of the script against the
shared dist/. Keeps the delegation surface narrow and the prune
logic single-sourced per shell.

[skip-tdd: wrapper] — same carve-out as install-side, smoke-test
evidence: fake dist/fake-stale-{sh,ps}.skill files created in real
dist/, ran build.{sh,ps1} --prune, verified fakes removed, real
caveman.skill etc. left intact.

Wiki .wiki/concepts/install-cross-platform.md extended:
- title broadened (Install → Install / Build)
- new "Build-side --prune" section
- scope note: dist-hermes/ is separate (managed by build-hermes.py)

Closes [install-ps1-build-prune-followup].

scripts/build.ps1

@@ -1,5 +1,9 @@
 # Build .skill archives from skills/<name>/ into dist/<name>.skill
-# Usage: build.ps1 [-Names <name1>,<name2>]   (no args = all)
+# Usage: build.ps1 [-Names <name1>,<name2>] [-Prune]
+#   no args        = build all skills/* into dist/<name>.skill
+#   -Prune         = after build, remove dist/<name>.skill files whose <name>
+#                    is NOT in skills/* (analogue of install.ps1 -Prune).
+#                    Prune always scans the full dist/, ignores -Names filter.
 #
 # Uses .NET System.IO.Compression.ZipArchive directly to produce
 # spec-compliant ZIPs with forward-slash entry names (Windows PowerShell 5.1's
@@ -7,7 +11,8 @@
 
 [CmdletBinding()]
 param(
-    [string[]]$Names = @()
+    [string[]]$Names = @(),
+    [switch]$Prune
 )
 
 $ErrorActionPreference = 'Stop'
@@ -69,3 +74,15 @@ foreach ($name in $Names) {
     New-SkillArchive -SkillName $name -SourceDir $srcDir -OutPath $out
     Write-Host "built: dist/$name.skill"
 }
+
+if ($Prune) {
+    $sourceNames = @(Get-ChildItem -Path $src -Directory | Select-Object -ExpandProperty Name)
+    $skillArchives = Get-ChildItem -Path $dist -Filter "*.skill" -File -ErrorAction SilentlyContinue
+    foreach ($archive in $skillArchives) {
+        $archiveName = [System.IO.Path]::GetFileNameWithoutExtension($archive.Name)
+        if ($sourceNames -notcontains $archiveName) {
+            Write-Host "pruning: $archiveName (not in skills/) -> $($archive.FullName)"
+            Remove-Item -Force $archive.FullName
+        }
+    }
+}