docs(phaseA): fix Windows smoke artifact-source to match build cmd

The Windows manual smoke used --guest-artifact-source
'C:\ci\workspace\artifact.txt' (a stale absolute path that does not
exist in the build flow; workdir is C:\CI\build). The build command
writes artifact.txt into the workdir, so the source must be relative
'artifact.txt' — matching the Linux example.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plans/PhaseA-user-checklist.md

@@ -178,7 +178,7 @@ Valida A3+A4 insieme: clone → wait → build → collect → cleanup.
     --repo-url 'http://10.10.20.11:3100/Simone/your-repo.git' `
     --branch main `
     --build-command 'echo hello > artifact.txt' `
-    --guest-artifact-source 'C:\ci\workspace\artifact.txt' `
+    --guest-artifact-source 'artifact.txt' `
     --artifact-base-dir 'F:\CI\Artifacts\smoke-job-win'
   ```