feat: add memory store and session manager

MEMORY.md for long-term memory, daily YYYY-MM-DD.md notes.
JSONL session files per chat_id with ring buffer history
(max 20 messages). All persisted on SPIFFS.

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

main/memory/session_mgr.c

@@ -0,0 +1,165 @@
+#include "session_mgr.h"
+#include "mimi_config.h"
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <dirent.h>
+#include <time.h>
+#include "esp_log.h"
+#include "cJSON.h"
+
+static const char *TAG = "session";
+
+static void session_path(const char *chat_id, char *buf, size_t size)
+{
+    snprintf(buf, size, "%s/tg_%s.jsonl", MIMI_SPIFFS_SESSION_DIR, chat_id);
+}
+
+esp_err_t session_mgr_init(void)
+{
+    ESP_LOGI(TAG, "Session manager initialized at %s", MIMI_SPIFFS_SESSION_DIR);
+    return ESP_OK;
+}
+
+esp_err_t session_append(const char *chat_id, const char *role, const char *content)
+{
+    char path[64];
+    session_path(chat_id, path, sizeof(path));
+
+    FILE *f = fopen(path, "a");
+    if (!f) {
+        ESP_LOGE(TAG, "Cannot open session file %s", path);
+        return ESP_FAIL;
+    }
+
+    cJSON *obj = cJSON_CreateObject();
+    cJSON_AddStringToObject(obj, "role", role);
+    cJSON_AddStringToObject(obj, "content", content);
+    cJSON_AddNumberToObject(obj, "ts", (double)time(NULL));
+
+    char *line = cJSON_PrintUnformatted(obj);
+    cJSON_Delete(obj);
+
+    if (line) {
+        fprintf(f, "%s\n", line);
+        free(line);
+    }
+
+    fclose(f);
+    return ESP_OK;
+}
+
+esp_err_t session_get_history_json(const char *chat_id, char *buf, size_t size, int max_msgs)
+{
+    char path[64];
+    session_path(chat_id, path, sizeof(path));
+
+    FILE *f = fopen(path, "r");
+    if (!f) {
+        /* No history yet */
+        snprintf(buf, size, "[]");
+        return ESP_OK;
+    }
+
+    /* Read all lines into a ring buffer of cJSON objects */
+    cJSON *messages[MIMI_SESSION_MAX_MSGS];
+    int count = 0;
+    int write_idx = 0;
+
+    char line[2048];
+    while (fgets(line, sizeof(line), f)) {
+        /* Strip newline */
+        size_t len = strlen(line);
+        if (len > 0 && line[len - 1] == '\n') line[len - 1] = '\0';
+        if (line[0] == '\0') continue;
+
+        cJSON *obj = cJSON_Parse(line);
+        if (!obj) continue;
+
+        /* Ring buffer: overwrite oldest if full */
+        if (count >= max_msgs) {
+            cJSON_Delete(messages[write_idx]);
+        }
+        messages[write_idx] = obj;
+        write_idx = (write_idx + 1) % max_msgs;
+        if (count < max_msgs) count++;
+    }
+    fclose(f);
+
+    /* Build JSON array with only role + content */
+    cJSON *arr = cJSON_CreateArray();
+    int start = (count < max_msgs) ? 0 : write_idx;
+    for (int i = 0; i < count; i++) {
+        int idx = (start + i) % max_msgs;
+        cJSON *src = messages[idx];
+
+        cJSON *entry = cJSON_CreateObject();
+        cJSON *role = cJSON_GetObjectItem(src, "role");
+        cJSON *content = cJSON_GetObjectItem(src, "content");
+        if (role && content) {
+            cJSON_AddStringToObject(entry, "role", role->valuestring);
+            cJSON_AddStringToObject(entry, "content", content->valuestring);
+        }
+        cJSON_AddItemToArray(arr, entry);
+    }
+
+    /* Cleanup ring buffer */
+    int cleanup_start = (count < max_msgs) ? 0 : write_idx;
+    for (int i = 0; i < count; i++) {
+        int idx = (cleanup_start + i) % max_msgs;
+        cJSON_Delete(messages[idx]);
+    }
+
+    char *json_str = cJSON_PrintUnformatted(arr);
+    cJSON_Delete(arr);
+
+    if (json_str) {
+        strncpy(buf, json_str, size - 1);
+        buf[size - 1] = '\0';
+        free(json_str);
+    } else {
+        snprintf(buf, size, "[]");
+    }
+
+    return ESP_OK;
+}
+
+esp_err_t session_clear(const char *chat_id)
+{
+    char path[64];
+    session_path(chat_id, path, sizeof(path));
+
+    if (remove(path) == 0) {
+        ESP_LOGI(TAG, "Session %s cleared", chat_id);
+        return ESP_OK;
+    }
+    return ESP_ERR_NOT_FOUND;
+}
+
+void session_list(void)
+{
+    DIR *dir = opendir(MIMI_SPIFFS_SESSION_DIR);
+    if (!dir) {
+        /* SPIFFS is flat, so list all files matching pattern */
+        dir = opendir(MIMI_SPIFFS_BASE);
+        if (!dir) {
+            ESP_LOGW(TAG, "Cannot open SPIFFS directory");
+            return;
+        }
+    }
+
+    struct dirent *entry;
+    int count = 0;
+    while ((entry = readdir(dir)) != NULL) {
+        if (strstr(entry->d_name, "tg_") && strstr(entry->d_name, ".jsonl")) {
+            ESP_LOGI(TAG, "  Session: %s", entry->d_name);
+            count++;
+        }
+    }
+    closedir(dir);
+
+    if (count == 0) {
+        ESP_LOGI(TAG, "  No sessions found");
+    }
+}