Atom is a very good text editor that covers your requirements.

• Free and Open-Source
• Code highlighting for JavaScript, HTML, CSS, SCSS, and PHP. (Bonus for Python and Ruby)
• Code completion for the same languages I mentioned above.

Check. Atom is open source and it includes code highlighting for the languages you mentioned. Some auto-completion features are provided through packages.

• Either a built-in terminal, or the availability of a plugin for a terminal.

Take your pick.

• SFTP/FTP remote editing.

I am not sure about this one, but you mentioned you are working on Ubuntu. I usually just mount the resource using File → Connect to server in Nautilus and use Atom or other editors for working on the files. If you really want it to be managed by Atom, this package seems a good place to start.

• Ability to open a directory rather than a single file. (Both remotely and non-remotely.)

You can have one or more directories set as projects and you will see the files in the navigation pane. I often run atom . in the working directory I want to use. Here's what it looks like:

• If the program is an IDE it must be lightweight. (Not something heavy like Eclipse or Netbeans.)

Lightweight may mean different things to different people. Atom runs on Electron (based on Chromium). It is a bit slow to start and it might have issues with huge files, however I find it totally fine to work with on a system with 4 GB of RAM.

I would also be ecstatic if it could do version control and had the ability to push changes with Git.

Atom is developed by GitHub. The colors you see in the navigation pane represent the status of each file. I do not use it, but this package seems to provide nice Git functionalities with keyboard shortcuts.

Geany（發音為“ genie”）是GPL2， lightweight ，突出顯示了OP的指定語言，完成了代碼，具有內置終端（或選項卡式的插件）

FTP &目錄打開，否，但有些是-開發人員建議通過以下方式安裝遠程文件系統：保險絲或LUFS，其功能與FTP等相同。

版本控制（ git ， subversion 等）可以與 GeanyVC插件和 git-changebar以獲得更多 git 功能。