feat(llm): add LLM integration instructions and handler

* Serve LLM instructions at `/llm`
* Include markdown content for memory instructions
* Update README with LLM integration details
* Add tests for LLM instructions handler
* Modify database migrations to use GUIDs for thoughts and projects

README.md

@@ -59,6 +59,8 @@ Run the SQL migrations against a local database with:
 
 `DATABASE_URL=postgres://... make migrate`
 
+LLM integration instructions are served at `/llm`.
+
 ## Containers
 
 The repo now includes a `Dockerfile` and Compose files for running the app with Postgres + pgvector.