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.c

@@ -0,0 +1,59 @@
+#include "message_bus.h"
+#include "mimi_config.h"
+#include "esp_log.h"
+#include <string.h>
+
+static const char *TAG = "bus";
+
+static QueueHandle_t s_inbound_queue;
+static QueueHandle_t s_outbound_queue;
+
+esp_err_t message_bus_init(void)
+{
+    s_inbound_queue = xQueueCreate(MIMI_BUS_QUEUE_LEN, sizeof(mimi_msg_t));
+    s_outbound_queue = xQueueCreate(MIMI_BUS_QUEUE_LEN, sizeof(mimi_msg_t));
+
+    if (!s_inbound_queue || !s_outbound_queue) {
+        ESP_LOGE(TAG, "Failed to create message queues");
+        return ESP_ERR_NO_MEM;
+    }
+
+    ESP_LOGI(TAG, "Message bus initialized (queue depth %d)", MIMI_BUS_QUEUE_LEN);
+    return ESP_OK;
+}
+
+esp_err_t message_bus_push_inbound(const mimi_msg_t *msg)
+{
+    if (xQueueSend(s_inbound_queue, msg, pdMS_TO_TICKS(1000)) != pdTRUE) {
+        ESP_LOGW(TAG, "Inbound queue full, dropping message");
+        return ESP_ERR_NO_MEM;
+    }
+    return ESP_OK;
+}
+
+esp_err_t message_bus_pop_inbound(mimi_msg_t *msg, uint32_t timeout_ms)
+{
+    TickType_t ticks = (timeout_ms == UINT32_MAX) ? portMAX_DELAY : pdMS_TO_TICKS(timeout_ms);
+    if (xQueueReceive(s_inbound_queue, msg, ticks) != pdTRUE) {
+        return ESP_ERR_TIMEOUT;
+    }
+    return ESP_OK;
+}
+
+esp_err_t message_bus_push_outbound(const mimi_msg_t *msg)
+{
+    if (xQueueSend(s_outbound_queue, msg, pdMS_TO_TICKS(1000)) != pdTRUE) {
+        ESP_LOGW(TAG, "Outbound queue full, dropping message");
+        return ESP_ERR_NO_MEM;
+    }
+    return ESP_OK;
+}
+
+esp_err_t message_bus_pop_outbound(mimi_msg_t *msg, uint32_t timeout_ms)
+{
+    TickType_t ticks = (timeout_ms == UINT32_MAX) ? portMAX_DELAY : pdMS_TO_TICKS(timeout_ms);
+    if (xQueueReceive(s_outbound_queue, msg, ticks) != pdTRUE) {
+        return ESP_ERR_TIMEOUT;
+    }
+    return ESP_OK;
+}