feat(config): add ConfigResolver with CLI>file>default precedence

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ClaudeMailbox.Tests/Config/ConfigResolverTests.cs

@@ -0,0 +1,59 @@
+using ClaudeMailbox.Config;
+
+namespace ClaudeMailbox.Tests.Config;
+
+public sealed class ConfigResolverTests
+{
+    [Fact]
+    public void CliFlag_WinsOverFile()
+    {
+        var file = new FileConfig { Port = 1000 };
+        var cfg = ConfigResolver.Build(new[] { "--port", "9999" }, file);
+        Assert.Equal(9999, cfg.Port);
+    }
+
+    [Fact]
+    public void File_WinsOverDefault()
+    {
+        var file = new FileConfig { Port = 1000, Bind = "0.0.0.0", DbPath = "/tmp/x.db" };
+        var cfg = ConfigResolver.Build(Array.Empty<string>(), file);
+        Assert.Equal(1000, cfg.Port);
+        Assert.Equal("0.0.0.0", cfg.BindAddress);
+        Assert.Equal(Paths.Expand("/tmp/x.db"), cfg.DbPath);
+    }
+
+    [Fact]
+    public void Default_UsedWhenNeitherCliNorFile()
+    {
+        var cfg = ConfigResolver.Build(Array.Empty<string>(), new FileConfig());
+        Assert.Equal(DaemonConfig.DefaultPort, cfg.Port);
+        Assert.Equal(DaemonConfig.DefaultBindAddress, cfg.BindAddress);
+        Assert.Equal(Paths.DefaultDbPath(), cfg.DbPath);
+    }
+
+    [Fact]
+    public void Mixed_CliPort_FileDbPath_DefaultBind()
+    {
+        var file = new FileConfig { DbPath = "/tmp/mixed.db" };
+        var cfg = ConfigResolver.Build(new[] { "--port", "7000" }, file);
+        Assert.Equal(7000, cfg.Port);
+        Assert.Equal(DaemonConfig.DefaultBindAddress, cfg.BindAddress);
+        Assert.Equal(Paths.Expand("/tmp/mixed.db"), cfg.DbPath);
+    }
+
+    [Fact]
+    public void CliDbPath_ExpandsEnvVars()
+    {
+        var file = new FileConfig();
+        var cfg = ConfigResolver.Build(new[] { "--db-path", "~/foo.db" }, file);
+        Assert.DoesNotContain("~", cfg.DbPath);
+    }
+
+    [Fact]
+    public void InvalidPortFlag_FallsBackToFileOrDefault()
+    {
+        var file = new FileConfig { Port = 4242 };
+        var cfg = ConfigResolver.Build(new[] { "--port", "not-a-number" }, file);
+        Assert.Equal(4242, cfg.Port);
+    }
+}