fix(ci): replace JS-based Renovate action with direct container invocation

The runner environment does not have Node.js in PATH, causing
actions/checkout@v4 and renovatebot/github-action@v41.0.0 (both JS
actions) to fail at startup.

Use renovate/renovate:latest as the job container and invoke the
renovate binary directly via run:, consistent with how all other
workflows in this repo handle checkout and tooling. The Checkout step
was also unnecessary — Renovate manages its own git operations.

.gitea/workflows/renovate.yaml

@@ -8,17 +8,15 @@ on:
 jobs:
   renovate:
     runs-on: ubuntu-latest
+    container:
+      image: renovate/renovate:latest
     steps:
-      - name: Checkout
+      - name: Run Renovate
-        uses: actions/checkout@v4
+        run: renovate
-        with:
-          ref: ${{ github.ref_name }}
-
-      - name: Self-hosted Renovate
-        uses: renovatebot/github-action@v41.0.0
-        with:
-          token: ${{ secrets.RENOVATE_TOKEN }}
         env:
-          RENOVATE_ENDPOINT: https://gogs.tftsr.com/api/v3
+          RENOVATE_TOKEN: ${{ secrets.RENOVATE_TOKEN }}
           RENOVATE_PLATFORM: gitea
+          RENOVATE_ENDPOINT: https://gogs.tftsr.com/api/v3
+          RENOVATE_AUTODISCOVER: 'false'
+          RENOVATE_REPOSITORIES: '["sarman/tftsr-devops_investigation"]'
           RENOVATE_AUTOMERGE: 'false'