ci: use go 1.25 and pin GOTOOLCHAIN=local to fix covdata failure

go.mod requires go 1.25.6 but the workflow was requesting go-version: '1.24'.
With GOTOOLCHAIN=auto (the default), the runner auto-downloaded Go 1.25.6
which lacked the covdata sub-tool, causing the test job to fail with
'go: no such tool covdata' for all cmd/* packages.

Changes:
- Bump go-version from '1.24' to '1.25' to match the go.mod directive
- Add a 'Pin toolchain' step that sets GOTOOLCHAIN=local, preventing
  any further auto-upgrade surprises regardless of future go.mod bumps

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

.gitea/workflows/ci.yml

@@ -14,9 +14,12 @@ jobs:
 
       - uses: actions/setup-go@v5
         with:
-          go-version: '1.24'
+          go-version: '1.25'
           cache: true
 
+      - name: Pin toolchain
+        run: echo "GOTOOLCHAIN=local" >> $GITHUB_ENV
+
       - name: Vet
         run: go vet ./...