Use `golangci-lint fmt` to format code, replacing the previous custom
formatter tool. https://github.com/daixiang0/gci is used to order the
imports.
`make fmt` performs ~13% faster while consuming ~57% less cpu while
formatting for me.
`GOFUMPT_PACKAGE` is gone because it's using the builtin package from
golangci-lint.
Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>