docs(reanalyze): add DCE refactor plan

AGENTS.md

@@ -42,6 +42,7 @@ The Makefile’s targets build on each other in this order:
 - **Test early and often** - Add tests immediately after modifying each compiler layer to catch problems early, rather than waiting until all changes are complete
 
 - **Use underscore patterns carefully** - Don't use `_` patterns as lazy placeholders for new language features that then get forgotten. Only use them when you're certain the value should be ignored for that specific case. Ensure all new language features are handled correctly and completely across all compiler layers
+- **Avoid `let _ = …` for side effects** - If you need to call a function only for its side effects, use `ignore expr` (or bind the result and thread state explicitly). Do not write `let _ = expr in ()`, and do not discard stateful results—plumb them through instead.
 
 - **Be careful with similar constructor names across different IRs** - Note that `Lam` (Lambda IR) and `Lambda` (typed lambda) have variants with similar constructor names like `Ltrywith`, but they represent different things in different compilation phases.