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/config.rs

@@ -14,6 +14,9 @@ pub const DEFAULT_CONFIG_PATH: &str = "~/.config/owlen/config.toml";
 pub struct Config {
     /// General application settings
     pub general: GeneralSettings,
+    /// MCP (Multi-Client-Provider) settings
+    #[serde(default)]
+    pub mcp: McpSettings,
     /// Provider specific configuration keyed by provider name
     #[serde(default)]
     pub providers: HashMap<String, ProviderConfig>,
@@ -48,6 +51,7 @@ impl Default for Config {
 
         Self {
             general: GeneralSettings::default(),
+            mcp: McpSettings::default(),
             providers,
             ui: UiSettings::default(),
             storage: StorageSettings::default(),
@@ -202,6 +206,21 @@ impl Default for GeneralSettings {
     }
 }
 
+/// MCP (Multi-Client-Provider) settings
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+pub struct McpSettings {
+    #[serde(default)]
+    pub mode: McpMode,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum McpMode {
+    #[default]
+    Legacy,
+    Enabled,
+}
+
 /// Privacy controls governing network access and storage
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct PrivacySettings {