feat: add FreeRTOS message bus (inbound + outbound queues)

mimi_msg_t carries channel/chat_id/content between tasks.
Decouples input channels from agent loop and output dispatch.

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

main/bus/message_bus.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include "esp_err.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/queue.h"
+
+/* Channel identifiers */
+#define MIMI_CHAN_TELEGRAM   "telegram"
+#define MIMI_CHAN_WEBSOCKET  "websocket"
+#define MIMI_CHAN_CLI        "cli"
+
+/* Message types on the bus */
+typedef struct {
+    char channel[16];       /* "telegram", "websocket", "cli" */
+    char chat_id[32];       /* Telegram chat_id or WS client id */
+    char *content;          /* Heap-allocated message text (caller must free) */
+} mimi_msg_t;
+
+/**
+ * Initialize the message bus (inbound + outbound FreeRTOS queues).
+ */
+esp_err_t message_bus_init(void);
+
+/**
+ * Push a message to the inbound queue (towards Agent Loop).
+ * The bus takes ownership of msg->content.
+ */
+esp_err_t message_bus_push_inbound(const mimi_msg_t *msg);
+
+/**
+ * Pop a message from the inbound queue (blocking).
+ * Caller must free msg->content when done.
+ */
+esp_err_t message_bus_pop_inbound(mimi_msg_t *msg, uint32_t timeout_ms);
+
+/**
+ * Push a message to the outbound queue (towards channels).
+ * The bus takes ownership of msg->content.
+ */
+esp_err_t message_bus_push_outbound(const mimi_msg_t *msg);
+
+/**
+ * Pop a message from the outbound queue (blocking).
+ * Caller must free msg->content when done.
+ */
+esp_err_t message_bus_pop_outbound(mimi_msg_t *msg, uint32_t timeout_ms);