hippocampus: move MemoryNode + store ops to where they belong

MemoryNode moved from agent/memory.rs to hippocampus/memory.rs — it's
a view over hippocampus data, not agent-specific.

Store operations (set_weight, set_link_strength, add_link) moved into
store/ops.rs. CLI code (cli/graph.rs, cli/node.rs) and agent tools
both call the same store methods now. render_node() delegates to
MemoryNode::from_store().render() — 3 lines instead of 40.

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

src/hippocampus/mod.rs

@@ -5,6 +5,7 @@
 // consolidation (spaced repetition, interference detection, schema
 // assimilation).
 
+pub mod memory;
 pub mod store;
 pub mod graph;
 pub mod lookups;