Fix warden access proxy for catalog fetch commands with shell pipes.

resolve_fetch_command used shlex.split, which treated `|` as a literal
argument — breaking reuse-surface-hub-write-token (kubectl | base64 -d).
Piped commands now run via shell=True with inherited stdio for --fetch.

tests/test_doubles.py

@@ -107,8 +107,8 @@ def test_proxy_fetch_runs_fully_offline_against_double(tmp_path):
         fetch_command="bao kv get -field=<FIELD> <path_template>",
         exec_capable=True,
     )
-    argv = resolve_fetch_command(entry, field="API_KEY", path="platform/x/y/z")
+    resolved = resolve_fetch_command(entry, field="API_KEY", path="platform/x/y/z")
     env = dict(os.environ, PATH=doubles_path_prepended(tmp_path))
     # proxy_fetch inherits stdout; run it in a child so we can capture the stream.
-    result = subprocess.run(argv, capture_output=True, text=True, env=env, check=True)
+    result = subprocess.run(resolved.argv, capture_output=True, text=True, env=env, check=True)
     assert result.stdout.strip().startswith(SYNTHETIC_PREFIX)