This is fun! I played this for longer than I expected I would. So, for the recommendations, I think that the skill tree could use some slight UI improvement with the logo colors. When the player comes back to the skill tree, it looks like some of the older skills are "greyed out" aka "maxxed out" (Even if there's more to unlock there!) so the player is more likely to run an ocean wide but an inch deep in the skills, by branching out to whatever the new shiny gold skills are, instead of putting more points into maxing out the skills towards the center (I hope that explanation makes sense). I played for awhile and couldn't beat the boss, but I'm sure I would have been able to easily progress past the boss challenge had I stayed even longer.
That all said, the strengths of this game is the skill tree, it keeps the player hooked in the game loop because progression feels so good. You come back with 200+ diamonds for your tree and you feel good about it because you always come back with even more. The gameplay mechanic itself is something new to me, you just align your cursor, and at first I wasn't sure how that would make an engaging game but I was very wrong! You do have to move the cursor to follow the enemies, but you also have to quick dart to catch your diamonds before they fall off screen, or else getting the enemies isn't worth it. So that is a good use of a very simple mechanic.
I wasn't sure where the agent theme comes in but I assume that is the more abstract use of the definition in the actual gameplay, where you are an "agent" of the slug? I guess in that sense, any game you are an "agent" of the character, but I think I see the connections in your game!
Overall, strong game! Good art and rewarding gameplay loop.