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/subconscious/mod.rs

@@ -26,6 +26,5 @@ pub mod defs;
 pub mod audit;
 pub mod consolidate;
 pub mod knowledge;
-pub mod enrich;
 pub mod digest;
 pub mod daemon;