Add lib target to poc-agent, make poc-memory depend on it

Split poc-agent into lib + bin so its API client, types, and tool
dispatch can be imported by poc-memory. This is the foundation for
replacing claude CLI subprocess calls with direct API calls to
vllm/OpenAI-compatible endpoints.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-agent/src/lib.rs

@@ -0,0 +1,11 @@
+// poc-agent library — reusable components for LLM agent work
+//
+// The binary (main.rs) is the full interactive agent with TUI.
+// This lib exposes the building blocks that other crates (poc-memory)
+// can use for their own agent loops.
+
+pub mod api;
+pub mod journal;
+pub mod types;
+pub mod tools;
+pub mod ui_channel;