Add docs/code-design.md with seven cross-cutting principles for
avoiding overengineering (one way to say one thing, behavior follows
from inputs, failures must reach a decision-maker, no seams without a
second consumer, spec-and-behavior drift, verify the path you fixed,
naming asymmetries). Reference it from AGENTS.md with the same weight
as in-file MUST/MUST NOT rules, mirroring the docs/terminology.md
pattern.
