pstack: index model-the-domain in poteto mode

Author: cursoragent

pstack/skills/poteto-mode/SKILL.md

@@ -48,6 +48,7 @@ Read the leaf skill in full for any principle you apply. Each entry names when i
 
 **Architecture**
 
+- **Model the Domain** (**principle-model-the-domain**). Writing stateful logic, or code that branches a lot or repeats a shape assumption across files. Encode the domain in a structure instead of scattered conditionals.
 - **Boundary Discipline** (**principle-boundary-discipline**). Wiring validation, error handling, or framework adapters. Guards at system boundaries, trust internal types, keep business logic pure.
 - **Type System Discipline** (**principle-type-system-discipline**). Designing types or a signature in any typed language. Make illegal states unrepresentable, brand primitives, parse external data at boundaries.
 - **Make Operations Idempotent** (**principle-make-operations-idempotent**). Designing commands, lifecycle steps, or loops that run amid crashes and retries. Converge to the same end state.