Clean up architecture specs: remove stale references, align docs with code, improve readability

- Replace stale DD references (DD3, DD6, DD9, DD10) with proper ADR links
- Fix 'Open Question 1' → OQ-01/OQ-03 cross-references
- Rewrite metagraph-module.md 'Why TypeBox Modules' to describe capabilities
  directly instead of framing as SchemaBuilder replacement
- Remove 'Transition from SchemaBuilder' section, replace with Source Structure
- Clean up implementation path: strikethrough phases → status table
- Fix data model diagram: remove non-existent nodeTypeId, fix EdgeType label
- Align EdgeConstraints examples with actual code (add default values)
- Clarify validateNode/validateEdge error behavior in docs
- Align EncryptedDataSchema code example with actual implementation
- Fix overview.md: correct dependency table, update current state, fix TypeBox URL
- Fix forward-look.md garbled text about dbtype element migration
- Fix open-questions.md: correct OQ count (4→7 open), add summary table
- Update doc statuses: schema-evolution, encrypted-data, open-questions → reviewed
- Update AGENTS.md to reflect current implementation state

docs/architecture/sqlite-host.md

@@ -1,6 +1,6 @@
 ---
 status: reviewed
-last_updated: 2026-05-29
+last_updated: 2026-05-30
 ---
 
 # SQLite Host
@@ -214,8 +214,7 @@ Standalone identity table. Currently not referenced by any relation — the
 `actors` table has no FK references to or from any metagraph table and is not
 included in `relations.ts`. This is a placeholder for identity data and may
 become a node type in an ACL graph (based on `@alkdev/operations`'s `Identity`
-interface) or remain a standalone table. See [overview.md](./overview.md) Open
-Question 1.
+interface) or remain a standalone table. See OQ-03 in [open-questions.md](./open-questions.md).
 
 | Column    | Type                | Constraints                             | Notes              |
 | --------- | ------------------- | --------------------------------------- | ------------------ |