fix(ci): install python3+PyYAML in the job when the runner image lacks them

Runner probe on PR #35 (run 461, log job 1979): checkout@v4 succeeds (network
fine) but the act container has node:20 + git 2.52 and NO python3 — 'python3
--version' exited 127, which was the failing check. Every validation step is
python3-based, so make step 2 self-provisioning via apt when missing.

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

.gitea/workflows/ci.yml

@@ -15,9 +15,19 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - run: python3 --version
+      - name: Ensure python3 available (runner image may lack it)
+        run: |
+          if command -v python3 >/dev/null 2>&1; then
+            python3 --version
+          else
+            . /etc/os-release
+            echo "python3 missing — installing on $ID $VERSION_CODENAME"
+            apt-get update -qq
+            apt-get install -y -qq python3 python3-yaml
+            python3 --version
+            python3 -c "import yaml" && echo "PyYAML OK"
+          fi
       - run: node --version
-      - run: echo "Runner works!"
 
       - name: Python syntax check
         run: |