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/agent/mod.rs

@@ -28,7 +28,6 @@ pub mod tools;
 pub mod ui_channel;
 pub mod journal;
 
-pub mod memory;
 pub mod runner;
 pub mod cli;
 pub mod context;