docs: make LEGO architecture attention-bounded

[iteathen]

Purpose

Make reasoning-context size a first-class LEGO boundary constraint across the portfolio.

Change

• defines the full-attention envelope as contract + implementation + invariants + lifecycle/resource/failure rules + tests/conformance + immediate dependency/consumer interfaces;
• requires recursive decomposition at real semantic/lifecycle/resource/substitution/change seams when that working set is too large;
• rejects arbitrary module splitting, duplicated truth, and cross-boundary internal knowledge;
• makes the hierarchy explicit: LEGO -> SOLID -> CUPID -> KISS;
• instructs repositories to carry the compact rule into mandatory local agent startup authority rather than relying on account-level discovery.

Documentation/governance only; no implementation or support claim changes.