feat(config): add FileConfig model and JSON loader

src/ClaudeMailbox/Config/FileConfig.cs

@@ -0,0 +1,41 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace ClaudeMailbox.Config;
+
+public sealed class FileConfig
+{
+    [JsonPropertyName("port")]
+    public int? Port { get; set; }
+
+    [JsonPropertyName("bind")]
+    public string? Bind { get; set; }
+
+    [JsonPropertyName("dbPath")]
+    public string? DbPath { get; set; }
+
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        PropertyNameCaseInsensitive = true,
+    };
+
+    public static FileConfig Load(string? explicitPath, string? defaultPath)
+    {
+        if (!string.IsNullOrEmpty(explicitPath))
+        {
+            if (!File.Exists(explicitPath))
+                throw new FileNotFoundException($"Config file not found: {explicitPath}", explicitPath);
+            return Parse(File.ReadAllText(explicitPath));
+        }
+
+        if (!string.IsNullOrEmpty(defaultPath) && File.Exists(defaultPath))
+            return Parse(File.ReadAllText(defaultPath));
+
+        return new FileConfig();
+    }
+
+    private static FileConfig Parse(string json)
+    {
+        return JsonSerializer.Deserialize<FileConfig>(json, Options) ?? new FileConfig();
+    }
+}

tests/ClaudeMailbox.Tests/Config/FileConfigTests.cs

@@ -0,0 +1,99 @@
+using ClaudeMailbox.Config;
+
+namespace ClaudeMailbox.Tests.Config;
+
+public sealed class FileConfigTests
+{
+    [Fact]
+    public void Load_ReturnsEmpty_WhenPathIsNullAndDefaultMissing()
+    {
+        var missing = Path.Combine(Path.GetTempPath(), $"nope-{Guid.NewGuid():N}.json");
+        var cfg = FileConfig.Load(explicitPath: null, defaultPath: missing);
+
+        Assert.Null(cfg.Port);
+        Assert.Null(cfg.Bind);
+        Assert.Null(cfg.DbPath);
+    }
+
+    [Fact]
+    public void Load_ReadsDefaultPath_WhenExplicitPathNull()
+    {
+        var path = WriteTemp("""{"port":9000,"bind":"0.0.0.0","dbPath":"C:\\tmp\\a.db"}""");
+        try
+        {
+            var cfg = FileConfig.Load(explicitPath: null, defaultPath: path);
+            Assert.Equal(9000, cfg.Port);
+            Assert.Equal("0.0.0.0", cfg.Bind);
+            Assert.Equal(@"C:\tmp\a.db", cfg.DbPath);
+        }
+        finally { File.Delete(path); }
+    }
+
+    [Fact]
+    public void Load_ExplicitPath_WinsOverDefault()
+    {
+        var defaultPath = WriteTemp("""{"port":1111}""");
+        var explicitPath = WriteTemp("""{"port":2222}""");
+        try
+        {
+            var cfg = FileConfig.Load(explicitPath: explicitPath, defaultPath: defaultPath);
+            Assert.Equal(2222, cfg.Port);
+        }
+        finally { File.Delete(defaultPath); File.Delete(explicitPath); }
+    }
+
+    [Fact]
+    public void Load_ExplicitPathMissing_Throws()
+    {
+        var missing = Path.Combine(Path.GetTempPath(), $"nope-{Guid.NewGuid():N}.json");
+        var ex = Assert.Throws<FileNotFoundException>(() =>
+            FileConfig.Load(explicitPath: missing, defaultPath: null));
+        Assert.Contains(missing, ex.Message);
+    }
+
+    [Fact]
+    public void Load_MissingFields_AreNull()
+    {
+        var path = WriteTemp("""{"port":1234}""");
+        try
+        {
+            var cfg = FileConfig.Load(explicitPath: path, defaultPath: null);
+            Assert.Equal(1234, cfg.Port);
+            Assert.Null(cfg.Bind);
+            Assert.Null(cfg.DbPath);
+        }
+        finally { File.Delete(path); }
+    }
+
+    [Fact]
+    public void Load_CaseInsensitive_PropertyNames()
+    {
+        var path = WriteTemp("""{"Port":1,"BIND":"x","DBPATH":"y"}""");
+        try
+        {
+            var cfg = FileConfig.Load(explicitPath: path, defaultPath: null);
+            Assert.Equal(1, cfg.Port);
+            Assert.Equal("x", cfg.Bind);
+            Assert.Equal("y", cfg.DbPath);
+        }
+        finally { File.Delete(path); }
+    }
+
+    [Fact]
+    public void Load_MalformedJson_Throws()
+    {
+        var path = WriteTemp("not json");
+        try
+        {
+            Assert.ThrowsAny<Exception>(() => FileConfig.Load(explicitPath: path, defaultPath: null));
+        }
+        finally { File.Delete(path); }
+    }
+
+    private static string WriteTemp(string content)
+    {
+        var p = Path.Combine(Path.GetTempPath(), $"mailbox-{Guid.NewGuid():N}.json");
+        File.WriteAllText(p, content);
+        return p;
+    }
+}