feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/core.dbml

@@ -0,0 +1,48 @@
+Table thoughts {
+  id bigserial [pk]
+  guid uuid [unique, not null, default: `gen_random_uuid()`]
+  content text [not null]
+  metadata jsonb [default: `'{}'::jsonb`]
+  created_at timestamptz [default: `now()`]
+  updated_at timestamptz [default: `now()`]
+  project_id uuid [ref: > projects.guid]
+  archived_at timestamptz
+}
+
+Table projects {
+  id bigserial [pk]
+  guid uuid [unique, not null, default: `gen_random_uuid()`]
+  name text [unique, not null]
+  description text
+  created_at timestamptz [default: `now()`]
+  last_active_at timestamptz [default: `now()`]
+}
+
+Table thought_links {
+  from_id bigint [not null, ref: > thoughts.id]
+  to_id bigint [not null, ref: > thoughts.id]
+  relation text [not null]
+  created_at timestamptz [default: `now()`]
+
+  indexes {
+    (from_id, to_id, relation) [pk]
+    from_id
+    to_id
+  }
+}
+
+Table embeddings {
+  id bigserial [pk]
+  guid uuid [unique, not null, default: `gen_random_uuid()`]
+  thought_id uuid [not null, ref: > thoughts.guid]
+  model text [not null]
+  dim int [not null]
+  embedding vector [not null]
+  created_at timestamptz [default: `now()`]
+  updated_at timestamptz [default: `now()`]
+
+  indexes {
+    (thought_id, model) [unique]
+    thought_id
+  }
+}