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/context_builder.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Build the system prompt from bootstrap files (SOUL.md, USER.md)
+ * and memory context (MEMORY.md + recent daily notes).
+ *
+ * @param buf   Output buffer (caller allocates, recommend MIMI_CONTEXT_BUF_SIZE)
+ * @param size  Buffer size
+ */
+esp_err_t context_build_system_prompt(char *buf, size_t size);
+
+/**
+ * Build the complete messages JSON array for LLM call.
+ * Combines session history + current user message.
+ *
+ * @param history_json    JSON array from session_get_history_json()
+ * @param user_message    Current user message text
+ * @param buf             Output buffer
+ * @param size            Buffer size
+ */
+esp_err_t context_build_messages(const char *history_json, const char *user_message,
+                                 char *buf, size_t size);