fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/channelserver/handlers_guild_scout.go

@@ -182,7 +182,7 @@ func handleMsgMhfAnswerGuildScout(s *Session, p mhfpacket.MHFPacket) {
 		bf.WriteUint32(guild.ID)
 		doAckBufSucceed(s, pkt.AckHandle, bf.Data())
 		for _, m := range mail {
-			m.Send(s, nil)
+			_ = m.Send(s, nil)
 		}
 	}
 }