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/004_create_thought_links.sql

@@ -1,7 +1,7 @@
 create table if not exists thought_links (
-  from_id uuid references thoughts(id) on delete cascade,
-  to_id uuid references thoughts(id) on delete cascade,
-  relation text not null,
+  from_id    bigint      not null references thoughts(id) on delete cascade,
+  to_id      bigint      not null references thoughts(id) on delete cascade,
+  relation   text        not null,
   created_at timestamptz default now(),
   primary key (from_id, to_id, relation)
 );