fix(skills): guard pruneSkillBackups against non-DWS directories

Restrict backup pruning to directories whose names match the DWS stamp
format (YYYYmmdd-HHMMSS with optional -N suffix) across all 8 installer
surfaces (Go, npm, 4 shell, 2 PowerShell). Unknown directories in
~/.dws/skill-backups are now preserved. Also fixes Windows coverage test
portability and covers the remaining macOS changed-code gap (retire
warning loop in runUpgrade).

.github/workflows/ci.yml

@@ -512,6 +512,10 @@ jobs:
         if: steps.classify.outputs.changelog_only != 'true' && steps.classify.outputs.docs_only != 'true'
         run: node .github/reviewer-routing.test.js
 
+      - name: Test npm installer smoke (prune, backup, publish)
+        if: steps.classify.outputs.changelog_only != 'true' && steps.classify.outputs.docs_only != 'true'
+        run: node test/scripts/install_js_smoke.mjs
+
   test-focused:
     name: "Test (focused: ${{ matrix.shard }})"
     needs: lint