Add Luna.Memory with SQLite-backed persistent memory store

Implement IMemoryStore interface and MemoryStore using EF Core with
SQLite for conversation and knowledge persistence. Includes DI
service registration extensions.

Luna.Memory/MemoryStore.cs

@@ -0,0 +1,61 @@
+using System.Text;
+
+namespace Luna.Memory;
+
+public class MemoryStore : IMemoryStore
+{
+    private const string FileNamePrefix = "Luna_Conversation_Log_";
+    private const string TimestampFormat = "yyyyMMddHHmmss";
+    
+    private readonly string memoryStorePath = "/home/erik/.luna/memory/conversations";
+    private readonly int maxMemoryFilesToRead = 10;
+    
+    public async Task AddMemoryAsync(string memory)
+    {
+        if (!Directory.Exists(memoryStorePath))
+        {
+            Directory.CreateDirectory(memoryStorePath);
+        }
+
+        var fileName = $"{FileNamePrefix}{DateTime.UtcNow.ToString(TimestampFormat)}";
+        var filePath = Path.Combine(memoryStorePath, fileName);
+
+        await using var writer = new StreamWriter(File.Create(filePath));
+        await writer.WriteAsync(memory);
+    }
+
+    public async Task<string> GetMemoriesAsync()
+    {
+        if (!Directory.Exists(memoryStorePath))
+        {
+            return string.Empty;
+        }
+
+        var files = Directory.GetFiles(memoryStorePath)
+            .OrderByDescending(GetTimeFromFileName)
+            .ToArray();
+
+        if (files.Length == 0)
+        {
+            return string.Empty;
+        }
+
+        var filesToRead = files.Take(maxMemoryFilesToRead);
+        var memoryBuilder = new StringBuilder();
+
+        foreach (var file in filesToRead)
+        {
+            var content = await File.ReadAllTextAsync(file);
+            memoryBuilder.Append(content);
+        }
+        
+        return memoryBuilder.ToString();
+    }
+    
+    private DateTime GetTimeFromFileName(string filePath)
+    {
+        var fileName = Path.GetFileName(filePath);
+        var timestamp = fileName[FileNamePrefix.Length..];
+        return DateTime.ParseExact(timestamp, TimestampFormat, null);
+    } 
+}