Workflows: replace actions/checkout with plain git

The runner failed resolving/running the external checkout action; a
git fetch of GITHUB_SHA with the workflow token needs neither action
resolution nor a node runtime in the job container, and works against
the private repo. Also document that a bare `cargo release` tags the
current version (the path for 0.1.0, which is already the workspace
version).

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

.gitea/workflows/release.yml

@@ -19,7 +19,14 @@ jobs:
       matrix:
         target: [x86_64-unknown-linux-gnu, aarch64-unknown-linux-gnu]
     steps:
-      - uses: actions/checkout@v4
+      # Plain git instead of actions/checkout — see ci.yml.
+      - name: checkout
+        run: |
+          git init -q .
+          git fetch --depth 1 \
+            "https://gitea:${{ secrets.GITHUB_TOKEN }}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git" \
+            "$GITHUB_SHA"
+          git checkout -q FETCH_HEAD
 
       - name: install tooling
         run: |