feat: add agent loop and context builder

Agent loop runs on Core 1: pops inbound messages, builds system
prompt from bootstrap files + memory, calls Claude API, saves
session, pushes response to outbound queue. PSRAM-allocated buffers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/agent/agent_loop.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Initialize the agent loop.
+ */
+esp_err_t agent_loop_init(void);
+
+/**
+ * Start the agent loop task (runs on Core 1).
+ * Consumes from inbound queue, calls Claude API, pushes to outbound queue.
+ */
+esp_err_t agent_loop_start(void);