docs(design): add principle about decomposing instead of early returning

Add a new design principle encouraging decomposition of functions instead
of relying on early returns, as early returns often signal mixed
responsibilities.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

context/design-principles.md

@@ -32,6 +32,12 @@ Commands compose via `Binding::Sequence` — a keymap entry can chain multiple
 commands, each contributing effects independently. The `o` key (add row + begin
 editing) is two commands composed at the binding level, not a monolithic handler.
 
+### Decompose rather than early return
+
+Early Returns usually are a signal of mixed responsibilities: if an early return 
+would clarify a function, consider how the function could be decomposed for the 
+same effect without the early return.
+
 ---
 
 ## 2. Polymorphism Over Conditionals