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/mcp/client.rs

@@ -0,0 +1,33 @@
+use super::{McpToolCall, McpToolDescriptor, McpToolResponse};
+use crate::{Error, Result};
+use async_trait::async_trait;
+
+/// Trait for a client that can interact with an MCP server
+#[async_trait]
+pub trait McpClient: Send + Sync {
+    /// List the tools available on the server
+    async fn list_tools(&self) -> Result<Vec<McpToolDescriptor>>;
+
+    /// Call a tool on the server
+    async fn call_tool(&self, call: McpToolCall) -> Result<McpToolResponse>;
+}
+
+/// Placeholder for a client that connects to a remote MCP server.
+pub struct RemoteMcpClient;
+
+#[async_trait]
+impl McpClient for RemoteMcpClient {
+    async fn list_tools(&self) -> Result<Vec<McpToolDescriptor>> {
+        // TODO: Implement remote call
+        Err(Error::NotImplemented(
+            "Remote MCP client is not implemented".to_string(),
+        ))
+    }
+
+    async fn call_tool(&self, _call: McpToolCall) -> Result<McpToolResponse> {
+        // TODO: Implement remote call
+        Err(Error::NotImplemented(
+            "Remote MCP client is not implemented".to_string(),
+        ))
+    }
+}