Fix three findings from final clean-pass audit

- apply.sh: gate sitecustomize.py install on backup success; a failed
  backup cp (disk full, read-only mount) previously fell through and
  overwrote the vendor file with no recovery path
- create_task.py: handle unexpected 2xx response schema in cmd_create;
  bare KeyError on result['id'] is replaced with a diagnostic print
- uninstall.sh: mv inside while loop had no error handling; under
  set -euo pipefail a failed mv aborted the script before rm -rf PATCH_DIR,
  leaving the system in partial-uninstall limbo

patch/create_task.py

@@ -179,7 +179,10 @@ def cmd_create(client, args):
     }
 
     result = client("POST", "/cloud_backup", body)
-    print(f"Created task id={result['id']}  name={result['description']!r}")
+    try:
+        print(f"Created task id={result['id']}  name={result['description']!r}")
+    except (KeyError, TypeError):
+        print(f"Task created but response schema was unexpected: {result}")
 
 
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