[plan] Unify code across multiple AI providers

_Automatically composed from the parent epic at dispatch time._

Split by architecture boundary: 9 coder children (9941–9949) in 4 phases (Foundation → Provider parity → Pi parity → Close-out). C4 gained a C2 edge; C9 reports EPIC INCOMPLETE rather than pass if a closing test is missing; AC5 tightened for phone width. Memory: none applied.