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/meta.dbml

@@ -0,0 +1,29 @@
+Table chat_histories {
+  id uuid [pk, default: `gen_random_uuid()`]
+  session_id text [not null]
+  title text
+  channel text
+  agent_id text
+  project_id uuid [ref: > projects.guid]
+  messages jsonb [not null, default: `'[]'`]
+  summary text
+  metadata jsonb [not null, default: `'{}'`]
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    session_id
+    project_id
+    channel
+    agent_id
+    created_at
+  }
+}
+
+Table tool_annotations {
+  id bigserial [pk]
+  tool_name text [unique, not null]
+  notes text [not null, default: '']
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+}