Add MVP Coordination section: code lives in citation-evidence umbrella during MVP

Documents the umbrella-first MVP decision (2026-05-24). This repo remains
INTENT-only until the engine's interfaces stabilize through real product
use. Points at the umbrella's wiki/SharedContracts.md, wiki/DependencyMap.md,
and docs/decisions/ as the canonical homes for cross-repo agreements.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

INTENT.md

@@ -318,6 +318,32 @@ This repository should be:
 
 ---
 
+## MVP Coordination — Code Lives Upstream
+
+During the umbrella-first MVP phase (decided 2026-05-24), **the source code
+for this subsystem does not live in this repository yet**. It lives in the
+umbrella repo at `citation-evidence/src/engine/` and `citation-evidence/src/shared/`.
+
+This INTENT.md documents the *intended* responsibilities and boundaries.
+When the engine's interfaces have stabilized through actual MVP use, the
+corresponding code extracts into this repository.
+
+**Shared contracts** (vocabulary, state enums, relation types, selector
+taxonomy, event types, viewer adapter, canonical text normalization, allowed
+dependency edges) are maintained in the umbrella repo:
+
+* `citation-evidence/wiki/SharedContracts.md`
+* `citation-evidence/wiki/DependencyMap.md`
+* `citation-evidence/docs/decisions/` (ADRs)
+
+This subsystem's eventual code must not contradict those documents. Changes
+to shared contracts happen in the umbrella, not here.
+
+Under the dependency map, **`citation-engine` is the leaf node** — it depends
+on none of the other subsystems. Every other subsystem depends on it.
+
+---
+
 ## Guiding Statement
 
 **citation-engine exists to make evidence-backed citations a stable, reusable, and portable domain model.**