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/lib.rs

@@ -37,7 +37,7 @@ pub mod memory_capnp {
 pub use hippocampus::{
     store, graph, lookups, cursor, query,
     similarity, spectral, neuro, counters,
-    transcript, memory_search, migrate,
+    transcript, memory_search, migrate, memory,
 };
 pub use hippocampus::query::engine as search;
 pub use hippocampus::query::parser as query_parser;