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>

Cargo.lock

@@ -2712,11 +2712,11 @@ dependencies = [
  "memmap2",
  "paste",
  "peg",
+ "poc-agent",
  "ratatui 0.29.0",
  "rayon",
  "redb",
  "regex",
- "reqwest",
  "rkyv",
  "serde",
  "serde_json",
@@ -3289,9 +3289,7 @@ dependencies = [
  "base64",
  "bytes",
  "encoding_rs",
- "futures-channel",
  "futures-core",
- "futures-util",
  "h2",
  "http",
  "http-body",