Point README at citation-evidence umbrella during MVP phase

Code lives upstream in citation-evidence/src/{shared,engine}/ during
the MVP. This README documents that, links the shared-contracts doc as
the authoritative source for type/state changes, and notes engine's
leaf-node position in the dependency map.

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

README.md

@@ -1 +1,19 @@
-Domain model and tooling for citations.
+# citation-engine
+
+Domain model and tooling for citations — the shared vocabulary
+(`Document`, `Selector`, `Annotation`, `EvidenceItem`, `EvidenceLink`,
+`EvidenceSet`, state enums, branded IDs, canonical text normalization)
+together with the services and repositories that act on it.
+
+## MVP status: INTENT only
+
+During the citation-evidence MVP, code lives upstream in
+[`citation-evidence`](../citation-evidence/) under `src/shared/` and
+`src/engine/`. This repo currently holds `INTENT.md` describing what will
+move here once the engine API stabilises. Until extraction, contract
+changes belong in
+[`citation-evidence/wiki/SharedContracts.md`](../citation-evidence/wiki/SharedContracts.md),
+not here.
+
+The dependency map (`citation-evidence/wiki/DependencyMap.md`) marks this
+package as a **leaf node**: nothing internal depends on anything else.