feat: reference field type (storage-less edges + admin UI)

.changeset/reference-field-admin.md

@@ -0,0 +1,5 @@
+---
+"@emdash-cms/admin": minor
+---
+
+Reference fields are now a real, working field type. Previously "reference" was just a plain text box with nowhere to point; now you get a proper relationship picker. Configure it in the schema editor (choose the target collection and single vs. multiple), then search for, pick, and reorder linked entries right in the entry editor — all saved together with the entry in one request. Referenced entries show a read-only "Referenced by" panel so you can see what points at them, and you can jump straight to any linked entry from the picker or the backlinks.

.changeset/reference-field-core.md

@@ -0,0 +1,5 @@
+---
+"emdash": minor
+---
+
+Reference fields now store real relationships between entries instead of an inert string. Selections are saved as first-class content-reference edges, written atomically with the entry in a single transaction, and hydrated on read alongside SEO and bylines — each resolved reference carries a readable display title (from the referenced entry's `title`, then `name`) so pickers and backlinks show a label instead of a slug. Reference fields are storage-less: they no longer add a column to a collection's table, and "Referenced by" backlinks come for free from the edge data. Seed files using a reference field apply its `$ref:` value as an edge (seed shape unchanged). Upgrade note: existing reference columns keep their data but are no longer written to.

demos/simple/emdash-env.d.ts

@@ -26,6 +26,7 @@ export interface Post {
   featured_image?: { id: string; src?: string; alt?: string; width?: number; height?: number; provider?: string; previewUrl?: string; meta?: Record<string, unknown> };
   content?: PortableTextBlock[];
   excerpt?: string;
+  relevant_posts?: string;
   createdAt: Date;
   updatedAt: Date;
   publishedAt: Date | null;