MCP-Client fuer HTTP-Server: externe Werkzeuge durch dieselben Pruefstellen (#197)

- Core/Mcp/McpClient: Streamable HTTP (MCP 2025-06-18) - initialize, tools/list
  with paging, tools/call; answers as JSON or event stream; session kept and
  opened again when the server forgets it; answers capped at 4 MB.
- McpServerTool: one server is one tool (mcp_<id>); its remote tools are the
  operations, so rights, preflight, staging, lead approvals, votes, audit and
  budget apply. On approval by default unless the human trusts an operation;
  reads foreign content; the server's descriptions are cut short and marked.
- McpConnections: registers, refreshes every 15 minutes, follows changes.
- InstanceConfig.mcpServers with protected headers; /api/mcp-servers;
  settings card "External MCP servers"; tool catalog includes the servers.
- stdio servers are not part of this (sandbox runner, separate step).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Core/Tools/ToolRegistry.cs

@@ -22,6 +22,13 @@ public sealed class ToolRegistry
             _tools[tool.Name] = tool;
     }
 
+    /// <summary>#197: Takes a tool out again — an MCP server the human removed.</summary>
+    public bool Unregister(string name)
+    {
+        lock (_lock)
+            return _tools.Remove(name);
+    }
+
     public IAgentTool? Get(string name)
     {
         lock (_lock)