feat(a2a-client): scaffold the A2A client extension behind A2AClient__Enabled

extensions/Volcanic.AgenticCore.Extensions.A2A/A2AClientSettings.cs

@@ -0,0 +1,23 @@
+namespace Volcanic.AgenticCore.Extensions.A2A;
+
+/// <summary>Section <c>A2AClient</c> (env <c>A2AClient__…</c>). Named so it doesn't clash with the SDK's <c>A2AClientOptions</c>.</summary>
+public sealed class A2AClientSettings
+{
+    public const string SectionName = "A2AClient";
+
+    /// <summary>Off by default: no <c>a2a</c> tool, no watches. Independent of the server's <c>A2A:Enabled</c>.</summary>
+    public bool Enabled { get; set; }
+
+    /// <summary>How long <c>send</c> waits for a result before a watch takes over. 0 hands over at once.</summary>
+    public int WaitSeconds { get; set; } = 60;
+
+    public TimeSpan WatchTimeout { get; set; } = TimeSpan.FromDays(1);
+    public int CardTimeoutSeconds { get; set; } = 10;
+    public long MaxFileBytes { get; set; } = 10 * 1024 * 1024;
+
+    /// <summary>
+    /// Our public base URL for push webhooks (<c>{PushBaseUrl}/a2a/push/{watchId}</c>). Set by the host from the server's
+    /// <c>A2A:PublicUrl</c> only while the server (and so the push receiver) is on; null = subscribe or poll.
+    /// </summary>
+    public string? PushBaseUrl { get; set; }
+}

extensions/Volcanic.AgenticCore.Extensions.A2A/Volcanic.AgenticCore.Extensions.A2A.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Description>A2A client extension: the a2a tool, remote agents in agents.json, watches for long tasks.</Description>
+  </PropertyGroup>
+  <ItemGroup>
+    <!-- Private="false": the host already ships Abstractions; an extension must not bring its own copy (docs/extensions.md). -->
+    <ProjectReference Include="../../src/Volcanic.AgenticCore.Abstractions/Volcanic.AgenticCore.Abstractions.csproj" Private="false" />
+    <PackageReference Include="A2A" Version="1.0.0-preview2" />
+  </ItemGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="Volcanic.AgenticCore.Extensions.Tests" />
+    <InternalsVisibleTo Include="Volcanic.AgenticCore.Telegram.Tests" />
+  </ItemGroup>
+</Project>