feat(mcp): add MCP client abstraction and feature flag

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.

crates/owlen-core/src/lib.rs

@@ -76,4 +76,7 @@ pub enum Error {
 
     #[error("Unknown error: {0}")]
     Unknown(String),
+
+    #[error("Not implemented: {0}")]
+    NotImplemented(String),
 }