move memory_search from hippocampus to subconscious/hook

memory_search.rs is agent orchestration (surface-observe, journal,
reflect cycles), not memory storage. Rename to hook.rs and move to
subconscious/ where it belongs.

Backward compat: pub use subconscious::hook as memory_search in lib.rs
so existing crate::memory_search paths still resolve.

src/session.rs

@@ -54,7 +54,7 @@ impl Session {
 
     /// Get the seen set for this session
     pub fn seen(&self) -> HashSet<String> {
-        super::hippocampus::memory_search::load_seen(&self.state_dir, &self.session_id)
+        super::subconscious::hook::load_seen(&self.state_dir, &self.session_id)
     }
 
     /// Get transcript metadata, resolving the path if needed.