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.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize session manager.
+ */
+esp_err_t session_mgr_init(void);
+
+/**
+ * Append a message to a session file (JSONL format).
+ * @param chat_id   Session identifier (e.g., "12345")
+ * @param role      "user" or "assistant"
+ * @param content   Message text
+ */
+esp_err_t session_append(const char *chat_id, const char *role, const char *content);
+
+/**
+ * Load session history as a JSON array string suitable for LLM messages.
+ * Returns the last max_msgs messages as:
+ * [{"role":"user","content":"..."},{"role":"assistant","content":"..."},...]
+ *
+ * @param chat_id   Session identifier
+ * @param buf       Output buffer (caller allocates)
+ * @param size      Buffer size
+ * @param max_msgs  Maximum number of messages to return
+ */
+esp_err_t session_get_history_json(const char *chat_id, char *buf, size_t size, int max_msgs);
+
+/**
+ * Clear a session (delete the file).
+ */
+esp_err_t session_clear(const char *chat_id);
+
+/**
+ * List all session files (prints to log).
+ */
+void session_list(void);