Add a `go vet ./...` step that runs before `go test -race ./...` in the
CI pipeline. This catches format string errors, unreachable code, and
other static analysis issues early.
Verified locally: `go vet ./...` exits 0 with no warnings.
Closesleeworks-agents/gitea-mobile#154
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>