Add model context protocol

[fuseraft]

Agents can now consume tools from any MCP server alongside built-in plugins. Servers are connected at session startup and torn down cleanly when the session ends.

• McpServerConfig model: stdio (Command/Args/Env/WorkingDir) and http (Url) transports
• OrchestrationConfig: new McpServers list
• McpSessionManager (IAsyncDisposable): connects servers, registers their tools as KernelPlugin via McpClientTool.AsKernelFunction()
• PluginRegistry: RegisterKernelPlugin / TryGetKernelPlugin for pre-built KernelPlugin entries (MCP or custom)
• AgentFactory: checks TryGetKernelPlugin before TryGet so MCP tools resolve transparently by name
• OrchestratorBuilder.Build → BuildAsync: initialises McpSessionManager before constructing agents
• RunCommand: await using var _ = mcpManager to guarantee disposal on all exit paths
• PluginsCommand: fuseraft plugins lists MCP server tools with an "MCP" type label
• README: new MCP section with config example and field reference