feat(cli): add MCP management subcommand with add/list/remove commands

Introduce `McpCommand` enum and handlers in `owlen-cli` to manage MCP server registrations, including adding, listing, and removing servers across configuration scopes. Add scoped configuration support (`ScopedMcpServer`, `McpConfigScope`) and OAuth token handling in core config, alongside runtime refresh of MCP servers. Implement toast notifications in the TUI (`render_toasts`, `Toast`, `ToastLevel`) and integrate async handling for session events. Update config loading, validation, and schema versioning to accommodate new MCP scopes and resources. Add `httpmock` as a dev dependency for testing.

crates/owlen-core/src/lib.rs

@@ -15,6 +15,7 @@ pub mod llm;
 pub mod mcp;
 pub mod mode;
 pub mod model;
+pub mod oauth;
 pub mod providers;
 pub mod router;
 pub mod sandbox;
@@ -36,6 +37,7 @@ pub use credentials::*;
 pub use encryption::*;
 pub use formatting::*;
 pub use input::*;
+pub use oauth::*;
 // Export MCP types but exclude test_utils to avoid ambiguity
 pub use llm::{
     ChatStream, LlmProvider, Provider, ProviderConfig, ProviderRegistry, send_via_stream,