fix(ci): use full GitHub URL for actions/checkout

Gitea Actions resolves bare action names against the local Gitea
instance by default. Using the full github.com URL makes the runner
pull the action from upstream.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       DOTNET_ROOT: /home/mika/.dotnet
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: https://github.com/actions/checkout@v4
         with:
           fetch-depth: 0