Add direct API backend for agent execution

When api_base_url is configured, agents call the LLM directly via
OpenAI-compatible API (vllm, llama.cpp, etc.) instead of shelling
out to claude CLI. Implements the full tool loop: send prompt, if
tool_calls execute them and send results back, repeat until text.

This enables running agents against local/remote models like
Qwen-27B on a RunPod B200, with no dependency on claude CLI.

Config fields: api_base_url, api_key, api_model.
Falls back to claude CLI when api_base_url is not set.

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

poc-memory/src/agents/mod.rs

@@ -16,6 +16,7 @@
 //   transcript   — shared JSONL transcript parsing
 
 pub mod transcript;
+pub mod api;
 pub mod llm;
 pub mod prompts;
 pub mod defs;