fix(test): stop overriding host-aware vitest scheduling in prepare gates (#67213)

The hardcoded `OPENCLAW_VITEST_MAX_WORKERS=4` default in gates.sh
short-circuits the host-aware scheduling introduced in c247e366.
`resolveLocalVitestScheduling` sees the explicit override and returns
maxWorkers=4, which falls below the >= 5 threshold required by
`shouldUseLargeLocalFullSuiteProfile`, so every machine—regardless of
resources—gets the DEFAULT profile (4 shard parallelism) instead of
the LARGE profile (10 shard parallelism).

Drop the hardcoded default so `test-projects.mjs` can detect actual
host resources and pick the appropriate profile automatically. When
the user explicitly sets OPENCLAW_VITEST_MAX_WORKERS, forward it as
before.

scripts/pr-lib/gates.sh

@@ -103,11 +103,16 @@ prepare_gates() {
       echo "Docs-only change detected with high confidence; skipping pnpm test."
     else
       gates_mode="full"
-      echo "Running pnpm test with OPENCLAW_VITEST_MAX_WORKERS=${OPENCLAW_VITEST_MAX_WORKERS:-4}."
-      run_quiet_logged \
-        "pnpm test" \
-        ".local/gates-test.log" \
-        env OPENCLAW_VITEST_MAX_WORKERS="${OPENCLAW_VITEST_MAX_WORKERS:-4}" pnpm test
+      if [ -n "${OPENCLAW_VITEST_MAX_WORKERS:-}" ]; then
+        echo "Running pnpm test with OPENCLAW_VITEST_MAX_WORKERS=$OPENCLAW_VITEST_MAX_WORKERS."
+        run_quiet_logged \
+          "pnpm test" \
+          ".local/gates-test.log" \
+          env OPENCLAW_VITEST_MAX_WORKERS="$OPENCLAW_VITEST_MAX_WORKERS" pnpm test
+      else
+        echo "Running pnpm test with host-aware scheduling defaults."
+        run_quiet_logged "pnpm test" ".local/gates-test.log" pnpm test
+      fi
       previous_full_gates_head="$current_head"
     fi
   fi