Have you tried writing into the AGENTS.md something like, "Always be on the lookout for dead code, copy-pasta, and other opportunities to optimize and trim the codebase in a sensible way."
In my experience, adding this kind of instruction to the context window causes SOTA coding models to actually undertake that kind of optimization while development carries on. You can also periodically chuck your entire codebase into Gemini-3 (with its massive context window) and ask it to write a refactoring plan; then, pass that refactoring plan back into your day-to-day coding environment such as Cursor or Codex and get it to take a few turns working away at the plan.
As with human coders, if you let them run wild "improving" things without specifically instructing them to also pay attention to bloat, bloat is precisely what you will get.