Eliminates the circular dependency between poc-agent and poc-memory by
moving all poc-agent source into poc-memory/src/agent/. The poc-agent
binary now builds from poc-memory/src/bin/poc-agent.rs using library
imports. All poc_agent:: references updated to crate::agent::.
poc-agent/ directory kept for now (removed from workspace members).
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
