Introduce the foundation for the Multi-Client Provider (MCP) architecture.
This phase includes:
- A new `McpClient` trait to abstract tool execution.
- A `LocalMcpClient` that executes tools in-process for backward compatibility ("legacy mode").
- A placeholder `RemoteMcpClient` for future development.
- An `McpMode` enum in the configuration (`mcp.mode`) to toggle between `legacy` and `enabled` modes, defaulting to `legacy`.
- Refactoring of `SessionController` to use the `McpClient` abstraction, decoupling it from the tool registry.
This lays the groundwork for routing tool calls to a remote MCP server in subsequent phases.