ci: use go-version-file to read version from go.mod (HAN-421)

Switch from hardcoded go-version: '1.24.0' to go-version-file: go.mod
so the CI Go version always stays in sync with the module's go directive.

All tests pass locally with GOFLAGS=-mod=vendor GOTOOLCHAIN=local.
Re-triggering CI run after previous run 1504 timed out on toolchain download.

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

.gitea/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
 
       - uses: actions/setup-go@v5
         with:
-          go-version: '1.24.0'
+          go-version-file: go.mod
           cache: false
 
       - name: Vet