ci: nanochat smoke on ailab gpu runner

- scripts/smoke.sh: end-to-end nanochat pipeline (dataset → tokenizer
  → tiny d=6 base_train, 50 iters); idempotent, caches under
  /data/nanochat-smoke (venv, uv-cache, shards).
- CN mirrors baked in: aliyun PyPI, sjtu pytorch-wheels/cu128, hf-mirror.
- workflow targets gpu label (ailab). drop earlier dummy workflow.

.gitea/workflows/smoke.yml

@@ -0,0 +1,15 @@
+name: smoke
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+  nanochat-smoke:
+    runs-on: gpu
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+      - name: nvidia-smi
+        run: nvidia-smi --query-gpu=name,memory.free,memory.used --format=csv
+      - name: smoke
+        run: bash scripts/smoke.sh