I am currently working on a tag wich returns a syntax highlightet code and wanted to use geshi to achive that, but I don’t know where I should put the geshi-specific files (since site/tags is for tags). Is there a folder to place those or can I safely put them in tags?
You can put it in site/plugins, where it is automatically loaded by Kirby and available from your tag. If you don’t want to load it at every request, you can just put it anywhere (the previous posters are both correct).