perf(ci): settle on capacity 2/node x 3 workers = 6 concurrent slots

capacity=3/node (9 total) oversubscribed the 8 physical cores per
node - individual builds slowed roughly 3x under contention (observed
directly: builds that took ~2m37s standalone were still running after
8+ minutes), for no clear net throughput win since this workload is
CPU-bound. Also found and fixed the actual reason capacity wasn't
taking effect at all initially: the runner daemon command never
passed --config, so it was silently running on default settings
regardless of config.yaml. Settled on 2/node (6 total, matching
docker-ready runner-2/3/4 - the control-plane node nxtgauge-1 is
deliberately excluded from the runner DaemonSet and should stay that
way, not used for build load).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/build.yaml

@@ -15,7 +15,7 @@ jobs:
     runs-on: docker-ready
     strategy:
       fail-fast: false
-      max-parallel: 9
+      max-parallel: 6
       matrix:
         service:
           - gateway