ci: gate census max-parallel 20 -> 24 for the ci-audit-planted behavior trials (Captain exception, GSTA-204)

A 3-trial behavior case needs three distinct slices; every gate slice is full,
so its trials open two new slices per census arm (claude-opus 18, codex-sol 6:
24 rows against a cap of 20).

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

.github/workflows/evals-periodic.yml

@@ -205,7 +205,7 @@ jobs:
           set -euo pipefail
           for arm in $(jq -r '.[]' <<<"$ARMS"); do
             mapfile -t arm_env < <(bun --no-install run scripts/census-arms.ts --env "$arm")
-            env "${arm_env[@]}" EVALS_TIER=gate bun run scripts/test-paid-shards.ts --tier gate --emit-plan "/tmp/gate-census-plan/$arm/manifest.json" --slice-budget 420 --jobs 2 --skip-judges --max-parallel 20
+            env "${arm_env[@]}" EVALS_TIER=gate bun run scripts/test-paid-shards.ts --tier gate --emit-plan "/tmp/gate-census-plan/$arm/manifest.json" --slice-budget 420 --jobs 2 --skip-judges --max-parallel 24
           done
 
       # One executor per arm and planned slice, each with its slice's ceiling.
@@ -574,7 +574,7 @@ jobs:
     strategy:
       # Two file workers per slice, each retaining two in-file case workers.
       fail-fast: false
-      max-parallel: 20
+      max-parallel: 24
       matrix:
         include: ${{ fromJSON(needs.plan-slices.outputs.gate_matrix) }}
     steps: