feat: add agent context handoffs

Handoff.cs

@@ -0,0 +1,44 @@
+sealed class Handoff
+{
+    public static readonly string[] Kinds = ["message", "context", "delegate", "review", "broadcast"];
+    public const int HistoryLimit = 100;
+    public const int MaxInFlight = 128;
+
+    public required string Id { get; init; }
+    public required DateTimeOffset Time { get; init; }
+    public required string From { get; init; }
+    public required string[] Recipients { get; init; }
+    public required string Kind { get; init; }
+    public string? Title { get; init; }
+    public string? Instructions { get; init; }
+    public string? Message { get; init; }
+    public string Status { get; set; } = "preparing";
+    public string? Error { get; set; }
+    public string? Package { get; set; }
+    public List<HandoffDelivery> Deliveries { get; } = [];
+
+    public object Snapshot() => new
+    {
+        id = Id,
+        createdAt = Time,
+        from = From,
+        recipients = Recipients,
+        kind = Kind,
+        title = Title,
+        instructions = Instructions,
+        message = Message,
+        status = Status,
+        error = Error,
+        content = Package,
+        deliveries = Deliveries.Select(d => d.Snapshot()).ToArray()
+    };
+}
+
+sealed class HandoffDelivery
+{
+    public required string Agent { get; init; }
+    public string State { get; set; } = "pending";
+    public string? Error { get; set; }
+
+    public object Snapshot() => new { agent = Agent, status = State, error = Error };
+}