Hey @mathurin68, sorry for the delay.
We have developer's guide which might help get you started. For the specific questions:
1) You can make this change in your own version if you fork the code base (it's open source so the code is publicly available). This can be updated in the localization files (i18n/en.json; webapp/i18n/en.json).
2) This is an interesting idea and potentially useful for many users on multi-team deployments.
Would this be restricted to hashtag search, or would you see it being useful to support cross-team search in general?
Do you have any initial thoughts on how this would look like in the UI (how to distinguish search results between the "bug" channel in Team A and Team B).