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/skills.dbml

@@ -0,0 +1,42 @@
+Table agent_skills {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [unique, not null]
+  description text [not null, default: '']
+  content text [not null]
+  tags "text[]" [not null, default: `'{}'`]
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+}
+
+Table agent_guardrails {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [unique, not null]
+  description text [not null, default: '']
+  content text [not null]
+  severity text [not null, default: 'medium']
+  tags "text[]" [not null, default: `'{}'`]
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+}
+
+Table project_skills {
+  project_id uuid [not null, ref: > projects.guid]
+  skill_id uuid [not null, ref: > agent_skills.id]
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    (project_id, skill_id) [pk]
+    project_id
+  }
+}
+
+Table project_guardrails {
+  project_id uuid [not null, ref: > projects.guid]
+  guardrail_id uuid [not null, ref: > agent_guardrails.id]
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    (project_id, guardrail_id) [pk]
+    project_id
+  }
+}