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>
