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

migrations/007_embeddings_table.sql

@@ -1,7 +1,7 @@
 create table if not exists embeddings (
   id        bigserial   primary key,
   guid      uuid        not null default gen_random_uuid(),
-  thought_id uuid       not null references thoughts(id) on delete cascade,
+  thought_id uuid       not null references thoughts(guid) on delete cascade,
   model     text        not null,
   dim       int         not null,
   embedding vector      not null,