Why does `help` not list all commands? How would I even list all commands? How am I reasonably to find `RemoveCondition`?
Help lists the commands that work in the current context. So if you're docked with a colony RemoveCondition would show up, but it wouldn't be visible if you used Help while in combat. You can use 'help all' to list all commands.
Help's output should mention this; if it doesn't then a bug was introduced somewhere.
EDIT: I started reading the source code and noticed that `list` can `list commands`. I just find it odd that help doesn't mention this. Reading the code or javadoc is a bit of a high ask IMHO.
There are a few commands that can list the loaded commands in addition to whatever else they do. "List commands" is one of them, "Status" is another. But neither is the intended way to find out about new commands, Help is. I'll look into making "help all" more visible.
As for what things list can show, that's displayed with "help list". You should never need to look at the source to know how anything works. If you do, let me know and I'll try and make things clearer.

There's also something I don't fully get. But it's probably in the game's code, not the mod code. I see that you can add condition "Teran" to a world. And most others, Jungle, Barren, etc. Not Toxic though. HOWEVER, it does not change the world. I'm guessing that the world is just a tag, and changing the tag doesn't affect the visuals? But it creates this very odd scenario where I add `Teran` to a lifeless Barren world with no atmosphere. But, again. I'm guessing that the condition is supplied by the game's api, and changing a planet from Baren to Teran is outside the scope of the command.
EDIT 2:
Yep, looking in the save file. The planet type is just a `<type>` tag. You'd also need to change other stuff. For example barren worlds don't have `atmosphereColor`. And tagging a previously barren world as `Teran` with no `atmosphereColor` makes the planet still look barren. I think.
Changing a planet's appearance is somewhat complicated and would require RunCode at the moment. The tag is used by the game when generating the planet to add a bunch of tag-specific stuff like conditions and setting its appearance, and merely changing the tag mid-game won't run all that planet generation code again.
You can check out com/fs/starfarer/api/impl/campaign/procgen/StarSystemGenerator.java in starsector-core/starfarer.api.zip for an example of how planets are set up, but be warned that a) there's a lot of code that goes into generating planets, and b) in addition to adding anything that planet type needs, you'll also have to remove any of the old stuff that no longer applies.