Drop reference to nonexistent Architecture.md

The companion rationale document was never written; this spec stands alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/Architecture-Build-Spec.md

@@ -6,8 +6,6 @@ built independently. It is deliberately lighter on the supporting services (Matr
 Gitea, Vikunja, Outline, LiteLLM) and heaviest on the **agent library** and the
 **two agents**, which is where the real design work lives.
 
-For deeper rationale behind any decision here, see the companion `Architecture.md`.
-
 **Document history.** The Tiered Inference amendment has been folded into this
 document and no longer exists separately: its A-series components appear as A10–A16,
 its network and credential revisions in A5 and B3, and its revised build order as