fix: add cross-file Ref declarations for relspecgo merge

- Add explicit Ref: blocks at bottom of each DBML file for cross-file FK relationships
- files.dbml: stored_files → thoughts, projects
- skills.dbml: project_skills/project_guardrails → projects
- meta.dbml: chat_histories → projects
- Update Makefile to use the relspec merge workflow where applicable
- Regenerate 020_generated_schema.sql with proper cross-file constraints

Addresses review comment on PR #20

schema/skills.dbml

@@ -40,3 +40,7 @@ Table project_guardrails {
     project_id
   }
 }
+
+// Cross-file refs (for relspecgo merge)
+Ref: project_skills.project_id > projects.guid [delete: cascade]
+Ref: project_guardrails.project_id > projects.guid [delete: cascade]