Make the interrupted-install legs able to fail

The probe treated a present .desktop-install-in-progress marker as proof of a
repairable state, but the drivers seed it unconditionally and never clear it, so
REPAIRABLE was unconditional and FALSE_READY unreachable. The Windows leg had no
kill-landed guard, blanket continue-on-error, and no repair re-run; -SkipTorch
was silently dropped, since install.ps1 parses only --no-torch.

Judge the re-run by whether the backend boots, on both platforms. The log grep
matched the frontend build printing "up to date" and failed a leg whose venv
was fine.

.github/scripts/interrupted_install_probe.py

@@ -118,7 +118,11 @@ def main(argv: list[str]) -> int:
         (out / f"{label}.log").write_text(log, encoding = "utf-8", errors = "replace")
         say(label, "ok" if rc == 0 else "failed")
 
-    # The in-progress marker #7490 writes before spawning the installer.
+    # The in-progress marker #7490 writes before spawning the installer. RECORDED ONLY,
+    # never used as repair evidence: both interrupt drivers seed it before every install
+    # and deliberately never clear it, so it is true on every leg by construction. Using
+    # it in the verdict below would make REPAIRABLE unconditional and FALSE_READY -- the
+    # single outcome this workflow exists to catch -- unreachable.
     home = Path(os.environ.get("UNSLOTH_STUDIO_HOME") or (Path.home() / ".unsloth" / "studio"))
     say("install_in_progress_marker", (home / ".desktop-install-in-progress").exists())
 
@@ -198,7 +202,6 @@ def main(argv: list[str]) -> int:
         facts.get("verify_install") == "failed"
         or facts.get("desktop_runtime_check") == "failed"
         or facts.get("capabilities.studio_install_ok") is False
-        or facts.get("install_in_progress_marker") is True
         or not facts.get("cli_h_ok")
         or not facts.get("capabilities_ok")
     ):