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/memory_store.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize memory store. Ensures SPIFFS directories exist.
+ */
+esp_err_t memory_store_init(void);
+
+/**
+ * Read long-term memory (MEMORY.md) into buffer.
+ * @return ESP_OK on success, ESP_ERR_NOT_FOUND if file missing
+ */
+esp_err_t memory_read_long_term(char *buf, size_t size);
+
+/**
+ * Write content to long-term memory (MEMORY.md).
+ */
+esp_err_t memory_write_long_term(const char *content);
+
+/**
+ * Append a note to today's daily memory file (YYYY-MM-DD.md).
+ */
+esp_err_t memory_append_today(const char *note);
+
+/**
+ * Read recent daily memories (last N days) into buffer.
+ * @param days  Number of days to look back (default 3)
+ */
+esp_err_t memory_read_recent(char *buf, size_t size, int days);