ci: drop ISO artifact retention from 14 to 3 days

Hit GitHub Actions artifact storage quota (50GB Pro tier) at 26
artifacts × ~2.7GB = 42GB. Each push burns ~2.7GB; 14d retention
+ frequent iteration = inevitable quota exhaustion.

3-day retention covers QEMU + spare-laptop test cycles. For long-term
keep, attach to GH Releases on tag (PR #2 will wire that).

.github/workflows/build-iso.yml

@@ -173,7 +173,7 @@ jobs:
           path: |
             build/out/*.iso
             build/out/*.sha256
-          retention-days: 14
+          retention-days: 3
 
       - name: Upload build log on failure
         if: failure()