agent: add MemoryNode for direct store access

MemoryNode is the agent's live view of a loaded memory node — key,
content, links, version, weight. Operations (load, write, search,
mark_used) go directly through the store API instead of spawning
poc-memory subprocesses.

This is the foundation for context-aware memory: the agent can track
which nodes are loaded in its context window, detect changes, and
refresh regions when nodes are updated.

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

src/agent/mod.rs

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