remove dead transcript mining pipeline

Delete enrich.rs (conversation extraction), select_conversation_fragments,
mark_observation_done, format_segment, and the {{conversations}} placeholder.
Transcript processing is handled by observe/journal agents now.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/lib.rs

@@ -67,7 +67,7 @@ pub use hippocampus::query::parser as query_parser;
 pub use subconscious as agents;
 pub use subconscious::{
     llm, audit, consolidate, knowledge,
-    enrich, digest, daemon,
+    digest, daemon,
 };
 // Backward compat: memory_search moved from hippocampus to subconscious::hook
 pub use subconscious::hook as memory_search;