docs: delete metagraph.md, migrate data model into metagraph-module.md

The historical reference doc was exactly the confusing artifact we were
cleaning up. Its unique content (the three-level type system overview
and ASCII diagram) now lives in metagraph-module.md as an introductory
section. Everything else was redundant:

- Schema types → metagraph-module.md (Module entries)
- SchemaBuilder → metagraph-module.md (SchemaBuilder Equivalence section)
- Usage patterns → metagraph-module.md + encrypted-data.md (Module examples)
- Composite identity / attributes storage → sqlite-host.md (table definitions)
- Versioning → schema-evolution.md (thorough treatment)
- Ecosystem context → overview.md (Ecosystem Integration section)

All cross-references updated: AGENTS.md, sqlite-host.md, schema-evolution.md.

AGENTS.md

@@ -98,12 +98,12 @@ Key changes from the originals:
 See `docs/architecture/` for detailed specifications:
 
 - `overview.md` — Package purpose, exports, design decisions, open questions
-- `metagraph.md` — Core graph model, schema types, SchemaBuilder, attribute
-  storage
-- `metagraph-module.md` — Graph type definitions as TypeBox Modules (evolution
-  of metagraph.md), naming conventions, migration path
+- `metagraph-module.md` — Graph type definitions as TypeBox Modules, data model,
+  naming conventions, implementation path
 - `forward-look.md` — Connections to dbtype, graph pointers, ujsx universal IR
   pipeline
+- `schema-evolution.md` — How graph type schemas evolve, TypeBox Value.Diff/Patch/Cast
+  for schema change detection and data migration
 - `sqlite-host.md` — SQLite tables, relations, client factory, porting notes
 - `encrypted-data.md` — Encrypted data design (planned), crypto utility, node
   type modeling