ci: bump build+integration gate go-version 1.23->1.25 for setup-go@v6

setup-go@v6 defaults GOTOOLCHAIN=local, blocking the auto-upgrade from
the pinned 1.23 to go.mod's required 1.25. Bump the pin to match go.mod.

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

Author: claude

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
 
       - uses: actions/setup-go@v6
         with:
-          go-version: '1.23'
+          go-version: '1.25'
 
       # The local gate (`make ci`) is identical to this CI gate:
       #   build  -> go build ./...